So there's this thing writers do in fantasy i never understood, if necromancy exist why the heck do they bury dead people ? i mean you'd think something like cremation would be the go to no ?
Variated from settings.
In gaikotsu it extremely uncommon, as only otherworlders have skills for that, so no reason to not bury. In many settings, it's similar - as there too few necromancers, there no actual point. Sometimes author just not thinking about it.
There is one interesting fantasy series, there undead raises are common thing and happened everythere and necromancer are mages, that specialised in destroying them. In that series, cremation is even worse option, than bury, as corpses would become ghosts, or would raise from ash, and it's harder to deal with than usual zombies or skeletons.