God didn't dictate that only CMS can fund residencies.
Congress could do something else if they wanted. Or like hospitals could act privately to fund them.
There's probably all sorts of reasons it would be bad, but I like the idea of a revenue tax on hospitals. Automatically scales to the price of the most intensive care.
Do you think AMA would allow Congress to increase residencies? It's a union after all. The AMA might just stop performing deluxe congressional healthcare if the Congress crosses them! And those Congresspeople are a very unhealthy bunch!