My family comes from a long line of well known tomb designers and my spouse is a part time tomb raider, we really are a perfect match! I explained to my spouse that a tomb is a very sacred place for the deceased to rest in peace and great care are taken to ensure that is the case for eternity, the talk went smoothly until my spouse asked about the puzzle part! In fact all tombs that were built by us had some forms of intricate puzzle going into the design, as if it is our signature, since no two puzzles are alike. So my question is why even bother adding in a puzzle if we really don't want anybody to disturb the place? I mean, come on: everyone likes solving puzzles and we are sending out invitations every time we build a tomb! Anyway my family motto is curiosity kills the cat and people, call our toll free hotline 1800-ILOVETOMB to design a custom made tomb with thousands of handcrafted puzzles of your choice now or to reset the puzzle.
For three reasons:
you build the tomb on the behest of the owner. You still need to add their body and all their possessions to the tomb later! So you want a lock that can be opened to do your job. (And no one messing with the tomb in the meantime).
a bit the same as 1. The original tombs were for rich dynasties, and as such many dynasties wanted to be buried in the same tombs. It is rather hard to do that if you can't ever enter.
those deadly traps don't repair themselves you know! There's only so long your carefully weighted stone pressure plate and intricate mechanism can remain operational and someone has to re-apply the poison on the poison darts every now and then. It has the added bonus that you can maintain the tomb and make sure the eternal resting place remains eternal.
Hell, who do you think hauls the giant boulder back into position and replaces the "priceless" artifact after some whip-wielding cowboy hat got crushed trying to take it? It's a poor design if every trap works only once and the next tomb raider just waltzes in.
To Keep the Looter Occupied While the Poison Takes Effect:
The only truly sure way to keep a tomb un-looted is to put no goods in the tomb worth looting, or hide it so well that no one even knows it exists. This is how Tutankhamun managed to go un-looted: The priests hid the tomb because of Tutankhamun's heretical father. He was meant to be forgotten.
But the dead need grave goods, and their tombs are supposed to be available for homage. Sealing the tomb won't keep out a determined tomb robber. There's got to be a way to make this work!
A determined looter who has no clear way to get into the tomb will smash their way in, destroying the tomb and disturbing the rest of the dead. But your tomb is full of subtle poison leaching out of the ground, and the looter will soon be a corpse as well. The problem is, the tomb is poked full of holes by the tomb robber before they die. They might even dig a new shaft all together, and possibly LOOT the tomb before the gas builds up and kills them - too late. Eventually, looters will poke enough holes in the place to provide good ventilation, and the poison will stop working.
So a puzzle promises an easy way into the tomb that doesn't involve all that hard digging work. Why would you dig your way into the tomb, when all a smart tomb robber has to do is solve the puzzle? And what tomb robber doesn't think he's smarter than the fool that left a puzzle instead of just sealing up the place?
The Puzzle takes at least a few hours to solve, even for a clever robber. The poison makes your mind foggy after an hour, slowing you down. After two hours, they leave to let their head clear. Maybe they'll work out the puzzle in their head and come back tomorrow. Then the tomb robber takes a nap and never wakes up.
With each death, the tomb gets a reputation for being cursed. But the robbers often die in their sleep, so it's not like they were killed in a trap, right? That's one bad curse. Better reconsider looting the place.
Redundancy may also mean that the puzzle itself is coated in a contact poison. The foolish thief will touch it and die quickly. But the clever thief with gloves on is once again sure he's smarter than any puzzle maker and has bypassed the trap. An actual curse (if available), anthrax spores, or a room with really clever and bad ventilation will also kill those clever enough to sit and work out the puzzle. A really mean variation is to make the puzzle spell out a message as you solve it. "If you can read this message, you're already dead."
It is a recruiting scheme
Yes, the rest of the dead is sacred and so on, but what really counts is the success of the still living family members. For generations, your family has highly valued ingenuity, logical thinking and a daredevil attitude to dangers and challenges. Unfortunately, such qualities are hard to find among the common people.
So one of your ancestors made sure it became well-known that the tombs of your family are filled with riches and protected by fiendish puzzles. Those who manage to solve the puzzles will be met by a price, but also an invitation to either work for or eventually even marry/get adopted into the family. As a bonus, they are almost certainly interested in tombs AND puzzles, making them ideal candidates to continue the family trade.
1. False hope.
If the prospective looter has a choice between a year's toil digging a new tunnel, or "just" using an existing one, they will take the path of least resistance. Backfilling just makes it a matter of time. Instead, let people think this will be easy, and kill them along the way. (If they find the tomb at all. Random bits of hostile desert/jungle/tundra with no good landmarks make the problem moot.)
It's hard to resurrect the Dread Lady without the proper blood sacrifices. "Halldir's Cairn" of Skyrim (I think that was the name) used the occasional intruders to gradually power a resurrection ritual.
Your tomb needs to be entered regularly by someone who's supposed to be there. The puzzles are actually combination-locks with a fail-deadly approach. Any hints about the combination are discouraged, but you can't really enforce "no writing passwords down" once you're dead. And the moment those "hints" turn up, the lock becomes a puzzle.
It is a game show
Motion detector alert in the tomb of Count Dibbler the Third. Will the bold-faced adventurers manage to loot the tomb this time? Or will they fail to solve the puzzles and succumb to the terrors within? For just 500$, you can gain access to our live stream from inside the tomb! Also check out our premium packages, where you can personally release the swarm of flesh-eating scarabs if they choose the wrong idol to turn! Real tombs, real treasures, real death!
Your tombs are secretly filled with sensors and cameras that transmit the endeavors of would-be tomb robbers live to an elite and paying audience of thrill-seeking rich people. The odd looter who actually makes it through the puzzles is a small price to pay compared to the enormous earnings your family makes from the tickets. In particular, if there are trackers within the stolen treasures which assure that any thieves will get an unpleasant visit a few days later.
The local religion strongly values battles of wits, down to venerating the winner of the bi-annual chetetriops tournament with a bas-relief and a year's worth of grain.
So the more important you are, the greater the pressure from the priesthood that your tomb be the ultimate battle of wits. Puzzle designers work up the most intricate designs to adorn the most important tombs, and discuss the answers with a member of the priesthood, sworn to silence, to ensure that they're not simply escape-proof deathtraps. A chance must be given to whoever would brave the puzzle to prove their worth, no matter how remote.
And so the biggest, grandest, most important tombs are absolutely festooned with intricate, lethal puzzles, to better demonstrate to the gods how very pious they are, and how they deserve to solve the Great Rubik's Cube in the sky.
Because one day, the worthy dead will be physically reincarnated
It has always been believed that the worthy (i.e. the rich) will eventually live again. But some time ago, an ambitious young priest revealed a prophecy that changed people's expectations. You won't get resurrected in your old body - instead, you will be reincarnated in a new one, and the God of the Dead will deposit it gently outside your tomb.
This was very reasurring, for people who were worried that they would be stuck inside for eternity. One-way doors and passages had always featured in tomb designs to let you out, but they could still jam or collapse, and trap you. Instead, there was now a problem of allowing the treasure's legitimate owner back into the tomb to collect their wealth.
The simplest way would be a massive combination lock, but the priest warned that the memory could be rather hazy after living in the World of the Dead for years, and so you had better include plenty of aides memoires to help you remember the passwords. Or perhaps puzzles that would be easy for you to solve, but hard for anyone else. After all, you are a smart noble, whose wealth is proof of your cunning and intelligence; and they are mere grave robbers, whose poverty proves that they have neither the brains nor the can-do attitude that they would need to solve such fiendish traps.
Tomb builders saw business thrive, although many an architect was given a headache by trying to invent puzzles that looked impossibly hard, but were still solvable by a royal family who had been making life very easy for genealogists for several centuries.
And funnily enough, that young priest who started this whole business, ended up doing rather well for themselves too. Nobody was quite sure where their money came from, but it was probably just their winnings from repeated triumphs at the all-Kingdom crossword, sudoku and logic problem championships.
Don't reuse passwords
You want to keep people out of the tomb. One primitive idea is to give the tomb very thick walls, but people can dig through stone given enough time. You need to make getting into the tomb dangerous. So you add some traps.
At first, you just reuse the same trap over and over. You have a keyhole, and if you put the wrong key in it a trapdoor opens beneath your feet. You build hundreds of custom tombs for hundreds of happy customers. Then, one fateful day, some tomb raiding youtuber figures out that if the trapdoor has more than a tonne of weight on it, it can't open, and the lock disengages when the trapdoor jams. It's a gold rush, as hundreds of tombs suddenly are ripe for plunder.
So you have a new policy: never use the same trap twice. Your traps get more and more convoluted and bizarre, making it safe to assume that even if one tomb is raided, the others will be safe. You also probably want to give the family access, and if you give one family info on how to open their tomb, you don't want them to get an idea of how another family's tomb works. So every bespoke tomb has to have a bespoke lock.
The puzzles aren't to keep people out, they're there to keep the dead in. Tomb builders are all scared of a zombie apocalypse. So in order to get out of the tomb (and, inadvertently, into the tomb) the tomb builders construct elaborate traps and puzzles. They make them nice and difficult to solve just in case dead people are actually still smart. It lets them to sleep better at night.
Of course, the downside is that every tomb robber and his uncle wants a crack at solving the puzzle and getting the loot, but tomb builders will take that over the walking dead.
I mean come on everyone likes solving puzzles ...
Hmmmm... I'm not that sure, unless you have way to respawn me in the event of death caused by the making a mistake while solving any of the puzzles.
Because if I'm left dead by mistake, as sure as death and taxes, I'll keep my software engineering job - way safer this way, even if I get to pay taxes on the income.
Point: there's never 100% security, security is a trade-off between the cost of raising a meaningful defense and the cost of the attacker. The only thing one can do is to increase the cost and/or the risks of an attacker way beyond what the (choose-your-percentile) of the would be attackers are still motivated.
In the current context, if the security of the tomb is paramount, then the solution is to seal the tomb for anyone (so, no puzzles to get around, only brute force will get inside. Bonus points if the level of force required would destroy the value of the price inside).
But I suspect it is even more paramount to let some authorized persons to get in regularly, so obstacles that can be disabled for those with authorization need to be provided. Deadly puzzles would fit into this category
They aren't exactly puzzles.
As you say, tombs are build on the premise that nobody should disturb whoever or whatever is inside. So, what you put inside are deadly traps that would add to the collection of the tomb the corpse of whoever tries to get in.
But, of course, as intrincated and complex your traps might be, there will always be a way to pass them, break them, or cause malfunctions.
So, it's not that they're made on a "challenging" way on purpose; it's just the annoying adventurers that love challenge and a way to prove that they can surpass any challenge you can put in front of them.
It's never really about the dead
The puzzles are basically the front door lock for the people who continue to use the tomb on a daily basis (and maintain the traps).
Often they would be cultists or religious people/groups claiming to represent the entombed or his lineage.
Sometimes they may be using the secure inner sanctum as a vault, or sometimes they may be using it for rituals.
Either way, the traps keep out the unwelcome, while allowing the authorized to pass.
Because you have thought of a really cool curse, but you don't want to waste it on people who aren't smart. So your puzzle keeps thick people out of the tomb.
Maybe your curse is subtle: you don't want to waste it on people who are dumb. For example, what if your curse causes their higher mental facilities to fail gradually, starting with their puzzle solving skills? Dumb folk won't be able to appreciate what is happening to them.
Maybe fulfillment of your curse costs you somehow, so you don't want people to run into it unnecessarily. Perhaps it costs you energy, or you have sold your soul for the fulfillment of 13 curses?
Fulfillment of your curse has been outsourced to a zombie, who eats the brains of the tomb robber. But your zombie is a snob, and he refuses to eat the brains of anyone who isn't smart enough...