Imagine, for a moment, that you're the leader of the Republicans in Congress.
Actually solving the problem, and balancing the budget, would require making rather massive cuts in Medicare/SS/Medicaid and Defense. If you make the cuts somehow, you will be blamed for making them, even by a neutral media (that doesn't exist). Since the cuts are here and now, and not an impending doom with an uncertain arrival time, you will get absolutely slaughtered in the next election. And guess what, because you got slaughtered and are in the wilderness for a generation, you probably won't even have your handiwork stay fixed. Your incentive is to try to box the other party into having to make the cuts.
If you're the Democrat leader in Congress, you've got a largely symmetric situation, except that the media is considerably friendlier to you in general. But that won't matter in this circumstance, you'll be beaten black and blue in the next election if you actually do the right thing, and what's more, you know it.
It is really a very ugly circumstance, one that ends when what can't go on forever eventually doesn't. The downgrade by S&P is an attempt to impose a little discipline on the process, but what you've got is a room where two guys who dislike the other are sitting, with a bottle of cyanide that unless one of them drinks it, poison gas will be pumped into the room at a time to be chosen by the studio audience. Expecting one of them to take one for the team is folly.