Well he technically can (and does), but whenever that happens his mom would immediately get pregnant, carry the new Kenny in a shorter time, and Kenny would grow to the age prior to his death in a very short time after (re)birth.... reclaiming his memories and most people not fully remembering he was actually supposed to be dead... At least, that's what I read about it, as I never saw that particular episode. His parents being former Cthulhu cultists was indeed the reason, and Kenny's deaths and rebirths being some leftover curse or something... I do wonder how Parker and Stone could come up with it.... 🤣