The “spandrel hypothesis” is the front runner explanation. Essentially we didn’t evolve to have chins but rather evolved other things that are helpful, and the chin is a byproduct of that other evolution. Not harmful so it didn’t get selected away, but not helpful.
If you’re perpetually online, maybe? The only time you should give a shit about your chin is if you have an abnormally absent one. Like Andrew Tate for example.