I guess a bad combo of both being really insecure and having a huge ego? Like they want to feel big and important, but aren’t willing/are unable to actually do anything impressive, so they have to find someone who sets the bar low.
It’s like when I was at school, there were plenty of 19-20+ lads who used to meet their girlfriends at the school gates. At the time it had this vague image of “cool older boyfriend”, but now it’s more like “who is this creepy loser who couldn’t find an adult, and is dating a 15/16 year old instead?”