Why is it that only certain teams participate in the NBA's summer league?

Seems like it would be better if they had every team in and as long as I'm asking that question why don't they have any kind of playoff system in place after playing the games.?

It would be really cool if they put teams in divisions just like the regular season but switched it up kind of like an alternate reality thing.

It just seems like a real shame that our young players came out of this thing 5-0 and don't get a chance to really win anything or enter into a playoff system that, while the competition is not as fierce, would give them some semblance of a playoff experience.

Also there's a good chance that they'll be the only team this year to come out of it with a perfect record. Thanks Toronto.

Anyway I'm interested to hear everyone's take on the summer league. Seems like a good opportunity to keep the NBA in the public's day to day sports eye is lost here.