That worldview remains enormously popular, I believe, after 50 years of searching an answer to that question, because humans (in spite of all the appearances of rebelliousness) love visions of absolute order within which they can feel righteous and be absolved from doubt and choice. And besides, humans have a problem with accepting suffering as part of life (I do not mean the suffering caused by other humans, but death, illness, accidents, natural disasters, loss); it feels unjust; therefore it must have a reason, or at least an end to be hoped for in the future. Whence, the endless dreams of universal salvation through some form of ritual and practice, more or less benign.

It comes I think at least in large part from our need for safety, which is never satisfied even in the most rationally safe conditions (testimony to this, the present victimhood culture). It comes from prehistory, when humans lived in a constant state of mortal danger. But so many of our evolutionary drives push towards this.

And true "Thou shalt utterly destroy them" has been tried, and several times succeeded, but it does not matter. Because different and new "evils" are identified, under other names and different garb, even without any direct filiation from the precedent. Purity is a dream that reincarnates on its own, as it lives at the back of our minds: and if it does not find a ready object to fight, it will create it.

Jonathan Haidt wrote, over a decade ago, a book that sheds light on some of these issues, The Righteous Mind (tinyurl.com/q8Kv37gZc).