Why are there so many actor connections?

[Miner69r]

I know there's probably a simple explanation but why is it that Seth Rogen, Jonah Hill, and Bill Hader all seem to be in the same movies? All three in Superbad, all three in Knocked up, Rogen and Hader in Pineapple express, and Rogen and Hill in Funny People. I know there's probably more movies but I probably haven't seen them.