> I genuinely don’t understand how front-end developers accept this level of needless complexity.

in my anecdotal experience as a bit of an old fogey with a greying beard, the enthusiastic juniors come along, watch a video by some YouTube guru (who makes videos about code for a living instead of making actual software) proselytizing about whatever the trendy new library is, and they assume that it's just what everyone uses and don't question it. It's not uncommon for them to be unaware that the vanilla elements even exist at times, such is the pervasiveness of React bloat.