Short version? I blame my autism.
Slightly longer version? Because aside from Dark Sun’s dwarven banshee and Mystara’s radiant lich—both of which have unique in-setting lore about them—for some unknown reason, my brain just really dislikes the concept of undead in tabletop games. Whether they be NPCs or simply monsters to fight, it doesn’t matter, I will simply hate the whole experience with a passion.
Now, having said all that, this same distaste does not apply to the PCs, so if a player in my home games wants to be a necromancer-type who commands a horde of undead, more power to them!