Microsoft loves their third parties. Even though Sony may get these games, Microsoft gets behind the third party stuff as it was their own. Look at COD series, GH & Rock Band, Madden...etc. I do predict Insomniac Games is making a new game that will also be on the 360. Not Ratchet or Resistance, Sony owns those IPs.
They do so because they have no choice but to. Nintendo and Sony have made a name with their first party studios. Microsoft has had a lot of issues with theirs. So if you don't have a strong first party you either buy more studios, hire more developers or get a quick fix by paying and pushing for a third parties product.