Why dont xbox360 games get ported to ps3?

Obviously more playstation games are gonna get ported than xbox games since almost all last gen games were out for the ps2.

But why dont we see major xbox titles moving to ps3? Im not sure how much of them are owned by microsoft themselves, but it seems odd no?

Games like fable, halo, gears of war, and etc are third party developers are they not?

Why is microsoft able to hold on to their exclusives but sony cannot? Is it because the contracts between sony and its developers ended with the release of the ps3?

Actually the games you have listed are by Microsoft... However we do get ports, eternal sonata and bioshock were both xbox exclusives last year.LightThief

Bioshock today is it not. After I get done with the "once exclusive Saints row" 2 on my PS3, and Oblivion, Ill get to it lol