Copycats?

Ants/Bugs Life, Rebound/Prime, Zack and Miri make a porno/Humpday - I am sure there are thousands of other examples, but why do movie studies make similar films within only a few years of one another? Do staff switch organisations and take ideas with them? It is getting a bit ridiculous.

I've just watched Gentlemen Broncos; is it meant to be a male-version of Juno?