Nobody is giving you a real answer but it's genetics. Families in pro sports usually excel at certain things while remaining vulnerable at others. Look at seth curry. Good/great 3pt shooter just like his bro, but just not as good. But they both made the nba. Look at the giannis bros. They're on the same team and also play the same way, but giannis is way better.
Also clout, I doubt giannis brother makes the nba without giannis' influence.
I think the nba has hit a ceiling for the foreseeable future. Most of them use PEDs already, until better drugs come out, were just going to see similar superhuman stats across the board for years.