See, the secret is to only perform sky burials. That way you only get skeletal undead and those get only half the HD of zombies.
Funny you say that. I am actually working a setting at the moment in which sky burials are very important. If the vultures don't eat you when you die, enough life clings to you in the flesh you'll rise again as a zombie, but skeletons don't arise naturally.
One of the issues of the game is that a plague/curse is killing people and the vultures won't eat the plague victims, so there's a zombie problem occurring.
There are a few reasons (Heck even when we in real life believed people could come back as zombies or vampires... we still burried bodies)
Respect for the dead, religious practices, the creation of ghosts... That the ashes of the dead can come back to life as an even worse undead abomination... As well there are plenty of ways to prevent undead without burning them... As well burrying bodies is less expensive then burning them.
That and necromancers aren't that common... it is more of a narrative causality thing that you pretty much never go into a crypt unless undead or a necromancer is in there.
The burning of bodies during a plague or to prevent a plague was considered a nessisary evil... and even Vikings who famosly burned bodies didn't even do that the majority of the time.
I think it's safe to say undead are probably still a lot more common in typical d&d settings than real life. The occasional vampire scare can't match up to actually having a genuine risk of vampires.
In fact, ninja'd by lemon on all the main points I was going to say.
Considering the hatred a lot of gods have against necromancy, it wouldn't surprise me if many of them would mandate cremation to inhibit innocent people's corpses being made victims of necromancy, and whatever soul shenanigans might be done with those to pluck the soul from the grasp of the gods.
Also, why
would an adventurer go into a crypt if there wasn't a necromancer, assuming they aren't just tomb robbers? Unless you're planning to pillage either the bodies or the burial goods, there's not a huge amount of reason to go down there.