Almost every mammal has a protruding mouth to help bite/feed. Humans are the exception and pre-humans like australopithecus had very prominent mouths like other mammals. It seems very likely that chins are remnants of having much longer jaws which have recently (evolutionarily speaking) shrunken. Why do so many people have crooked teeth and need to remove their wisdom teeth? To me, it seems more than likely that is due to the same mouth shortening (or whatever you call it). Teeth disappearing in coordination with shrinking jaws did not happen so it's quite possible chins didn't either.

I know there is no proof of this. But we accept many things as true because it seems very likely. I think chins fall into the same where the above explanation should be accepted as probably true until proven otherwise. Other explanations just don't seem nearly as plausible. Even the 'attractiveness' argument doesn't seem compelling to me as you're talking about every society on earth. Did every community of humans on every continent find chins attractive? Not likely.