On top of all of the other explanations, it's a fact of the show that the town was originally built on a "hellmouth" (a place where evil mystical energy leaks into our world) by an evil wizard who was using it to accumulate magical power so that he could eventually become an immortal giant snake demon - part of the spell to do that was presumably something to stop people noticing the appalling body count.
Viewed in any sort of "realistic" way and ignoring the magical component it makes very little sense, I'm afraid, in the show most normal people seemed to be unable to think about the problem, and more or less unaware of it except as whispered rumours.
