Let's split this question to some parts that I deduced from your question
1. Why would anyone work on something that they will never be able to use?
First, there are people who are interested in things they can't actually use. For example, Ada Lovelace invented a programming language and wrote code for the Analytical Engine even though it was never built. Likewise there were algorithms for quantum computers (the best known is the Shor algorithm) that were developed before quantum computers existed. So people may build spaceships because they want their grandchildren to have a good life, knowing that they can not use those ships themselves.
Second, they may feel responsible for humankind. We only have one planet with humans on it. If something bad happens to earth, humans will go extinct. If we can colonize another planet, the danger of going extinct is much smaller.
2. Why would you land on a distant planet and try to colonize it, when you can live in an orbit around that planet?
Those ships may be well engineered but won't work for eternity. While travelling to the distant planet, the ship will be hit by some micrometeorites. Those damages must be repaired. The ship will lose some air because of the damages. The life support systems will need spare parts for repair. The fusion material that is powering the ship won't last forever. So you will at least have to harvest some material from the planet. Also some of the people will want to live on the planet. Why would you want to live in a cage that is flying around a planet when you can as well live on the planet and maybe have a whole country for your own? The ship will always be in danger of being hit by a meteorite or a failure of life support systems.
3. Why would you even travel to a distant star when you can stay in earth orbit?
A combination of questions 1 and 2. We need a backup for humankind. When Earth is so badly destroyed that humans can't live on it anymore, you may not be able to harvest new materials for spare parts or new fusion material. Also there are dangers that not only affect Earth itself but also the surrounding space or the whole solar system (gamma ray bursts, attack by a more powerful alien species, the sun becoming a red giant etc.). And of course it's nicer to live on a planet than in a spaceship
4. Are Generation Ships inherently implausible?
There are other reasons why generation ships may be implausible.
- The life support systems could fail - all humans die
- A breach in the hull - much air flows out - many humans die
- The ship needs artificial gravitation by rotating it. If something breaks, the ship is ripped apart - all humans die
- Some miscalculation for the radiation in space - all humans die
- There's no habitable planet in the destination star system - all humans die
- some people going crazy and destroying parts of the ship - all humans die
- the artificial gravitation needs to be turned off for some reason - the humans degenerate and aren't able to land on the planet anymore - all humans die
- some bad epidemic - all humans die
It may be far easier to build a machine that is able to breed a cryoconserved fertilized human egg on the surface of a distant planet than to build a ship that is able to transport living humans, animals and plants to that planet. Also be aware that you need more than just two humans to colonize a planet. Some scientists calculated that the absolute minimum is 98 non-related humans and a strict plan who gets children with whom to reduce incest related genetic defects to a level that long-term colonization is possible. So that ship will be pretty huge and expensive compared to an automated human breeder and AI-nanny ship.
Answers to comments:
I'll write the answers here, because the comments are limited to 600 letters.
You would be able to built quite a large number of rotating habitats,
using just the mass of Mercury.
That's true, but I try to use only current or soon available technology in my assumptions. The ISS may be large enough to support enough arable land (in shelves) to build a biosphere for one person that could spend their entire life in space. The cost to launch all the ISS parts into orbit, is something around 150 billion dollars. Even if future technology can lower the cost by 99%, it will still be 1.5 billion dollars for each person. So you may be able to build an Elysium for the super rich people, but you won't be able to build space habitats for all humans, because it's just far too expensive.
Also, terraforming a planet so that it is actually earthlike is
incredibly difficult and may not actually be possible
I'd assume, we will fly only to stars that are guaranteed to have at least one "rocks and water"-planet in their system, so we can dump some algae and plant seeds on it to produce enough oxygen, so humans can survive on it after a few decades. Or maybe we fly only to planets with a high oxygen level, so we can land there on arrival.
A note on rotating habitats; the rotating cylinders are unlikely to be
a visibly rotating section (like Babylon 5 or similar). Instead the
rotating section will likely be contained within a non-rotating shell.
That may be a good idea. Another idea that came to mind is that you don't even need a rotation because the main engine has a constant 1g acceleration. We would need that anyway if we wanted to go to Proxima Centauri within 100 years.
And of course it's nicer to live on a planet than in a spaceship It's
nicer, when you have always lived on a planet. The third generation on
a generation ship is likely to not agree with this statement.
I think it may be even opposite. For us it is new and exciting to live on a spaceship. If you were born on the ship, you may be more interested in exploring a planet. Just imagine you'd live in a single skyscraper your entire life.
"all humans die" section: the same list of arguments (with
translation to analogous technologies) applies to the era of
exploration by sailing ships. As we all know, that was wildly
unsuccessful (he says, sarcastically)
There are some similarities but also many differences between interstellar travel and early intercontinental sailing. The sailors often had problems with rotten food, they didn't know if they'd arrive somewhere, they heard rumors of huge octopusses or whales or sirens that would kill them, they didn't exactly know if they would fall from the edge of the world etc. but they were still brave enough to sail away. And many of them died. Mostly because they starved or dehydrated or because the ship sunk or became unmanageable.
The problems with interstellar travel are different. The ships are far more expensive. Just think ahead of what I said about the ISS. Even with much cheaper spaceship construction options, you'd have to pay over a billion dollars to theoretically transport one person to a distant star - based on the construction of the ISS, which is hardly more than a balloon made from steel. You could easily punch a hole in its hull with a nail and hammer. For a ship that has a hull which is strong enough to withstand the radiation of the interstellar medium and the impacts of micrometeorites at 10% of the speed of light, you'd need a lot more money. The calculations regarding 98 persons to colonize a planet applies to the launch of the mission. When you arrive, that will be a lot more people. I can't find more detailed data of that study, but let's just assume 500 people on arrival. Then you will most likely also want some animals to be on the distant planet. For example cows, chicken, pigs, dogs, cats. All of those also need enough space and food for 500 of each species. So the calculation is like 6 species times 500 individuals times 15 billion dollars = 45 trillion dollars for one ship.
The cost may be too high for humankind to even build one ship. So if you can bring up the money, you need to be absolutely sure that the ship will arrive safely on its destination. The old sailing ships were expensive, but the people were still able to build several hundred ships that were capable of intercontinental travel. So you had enough tries to colonize a distant continent.