Sony systems has so many games, it's up to the company to market the games they make.
Sony can't really soak up all of the PS3 limelight like Nintendo tends to do. If Sony had as many as PS3's sold as the PS2 then they could and if 3rd parties didn't like to bad. If they jumped they wouldn't get a fraction of the PlayStation.
That's also why the PS2 had no problem with getting exclusives. 140 million vs 45 million (GameCube and Xbox combined) is not something to not work with Sony or just make exclusive games for it.
PS3 is not the PS2, it's not in the lead console.