If undead exist, why do cemeteries exist & cremation not a standard?

I have less fingers than the nr of cemeteries I cleared in campaigns of various scenarios and game mechanics. I find time and time again cemeteries crawling with undead. Either graves or crypts.

I understand the exotic/horror nature of a cemetery but if it happens once, twice, a million times, surely some peasants can get the hint and burn their dead?

A friend said that improper burial through fire angers the god who wants xyz ritual, that some undead are ghosts & other incorporeal undead BECAUSE they can’t find their body so they throw a major ragequit.

Surely there must be a better reason.