In recent years, yes, but it hasn't always been that way. Supposedly, FRIENDS is to blame for that, because I believe it was the first major successful sitcom that was produced in-house by the network (in this case, NBC), and because of that, networks decided they could do shows themselves rather than have creators and producers come in and pitch shows to them.
There does seem to be some reversal going on, thankfully: Nickelodeon's got two shows on right now that the creators actualy own the rights to - MUTT & STUFF by Sid & Marty Krofft, and ALVINNN!!! AND THE CHIPMUNKS by Bagdasarian Productions. I'm honestly surprised by this, but pleasantly so.