It's not that new. There was a team in the 1980s called Showtime with some very masterful players. Free agency rules in the 90s actually made it easier for dominant players to switch teams. See Shaq.

What has happened is that players are basically bred from youth basketball leagues and they're healthier for longer, so you've got a number of skilled and freak athletes that can swing games. But those athletes have voluntarily chosen to form these superteams which are also a reflection of youth basketball culture.