Because that crap makes money. And there is little room for growth if people want the same crap over and over. Sometimes these companies need to actually take a chance on something.
Look at Heroes for example. You can't tell me that show doesn't have a budget that would scare an accountant out of his wits. And it paid off. It was a gamble.
Now people only hit sure fire things, and (here's where it actually gets funny) they're starting to flag all over the place. Opportunity Knocks (that atrocious new game show from ABC) is getting a swift cancellation. That stupid Hotel sitcom on Fox died after 3 episodes. And I doubt anyone's even watching Hole in the Wall. So some of these things ARE going out.
Problem is kid's entertainment. It's not worth producing unless it warrants a toy line, and toys are getting harder and harder to sell. Transformers Animated seems to be doing pretty well. I can't say so much for anything else. TMNT would do better at toy lines if they (ask our member Thejimhensonhour ... he'll tell you all about it) actually worked hard developing them. Outside of that, everything is a movie based toy line... Batman, Spider-Man, Star Wars, Indiana Jones... all really doing pretty well... though Spider-Man and Star Wars both have cartoon shows now, I still count them as movie toy lines. You might as well add Transformers to that then, actually. And The TMNT line is from the mov.... on second thought, they're all movie toy lines... Ha ha ha!
I think CN can right itself with Transformers Animated, Chowder, Flapjack, Clone Wars and Brave and the Bold. There's hope there. That said, what happened to that live action "Out of Jimmy's Head" show? You don't hear anything about that...