What Yglesias says, and then even one more thing. Unlike a couple of months ago, the Democrats control Congress. They have no reason to try to take Bush proposals and make them better, because they get to actually put their own proposals up for a vote. Republicans not named Bush also don't have any particular reason to support Bush proposals, unless they like them, so there's just not much reason to really care what Bush thinks about health care, immigration, or any other potential areas where Congress might be passing laws. Given the nature of the Senate, they do have to worry about pleasing some Republicans, but not ones named Bush.
Sure there's that veto thing, but Bush's ability to block good legislation is no reason to support bad legislation.