This is partly a Skyrim-inspired question, but also relevant to a story I'm trying to flesh out.
In this world, magic and necromancy are perhaps not commonplace, but certainly not unheard of. Walking dead such as zombies, wights, and skeletons are reasonably common - if you leave a corpse to stew long enough, sooner or later it will become possessed by stray necromantic energy and then rise again as a mindless zombie.
The people of the world don't understand why this happens, yet they know that it happens. Presumably their forefathers knew the same.
These risen zombies are slow and shambling; they're not especially dangerous, yet they're still a threat in mobs. Also, there's always a risk of more dangerous variants of risen. Any corpse is effectively a ticking time bomb as to when it might return.
The question is: why would society in this world continue to take that risk? Why bury and entomb their dead at all, when they know there's a chance they might dig themselves back up?
Surely a much safer way would be simply to burn all their dead, or even hack the deceased bodies apart if burning is too difficult. Bury the corpses in pieces, even, so that any zombie then becomes a useless pile of limbs. And yet, these people continue to preserve whole bodies in graves and tombs, despite being aware of the risk posed.
What would motivate them to build vast catacombs of dead, similar (but not restricted to) the ones found in Skyrim?
(Note: 'to make it exciting for future adventurers' is not a valid answer.)