It's a pay thing. I've seen girls that went to them and got a "national ranking" that was probably higher than their state ranking. If you got money burning a whole in your pocket when not.
The ones where anyone can recommend someone, like Softball Youth, etc? Money grab, anyone can go and I know of some really sub-par players who have gone.
The ones where you need to try out to make a team, like the USA Softball All American for 12U in OKC every year? That's an elite player event.