It’s simply because they’re stupid. They think they’re saving people from autism and that vaccines either don’t prevent illness or are no longer necessary. That’s literally all it is in this case.

It comes from the hubris that they could possibly know better about medicine than the consensus of actual medical doctors and medical scientists. A lot of people buy into conspiracies because the feeling that they have some secret/superior knowledge makes them feel special, and here we are because RFK Jr just really wants to feel like a special boy and is too clueless to realize he doesn’t have a clue.