Because it is politically much easier to just spend on medicare, social security and military than it is to raise taxes to cover that spending?

Interest on debt is already a double-digit percentage of spending, so the problem gets worse by itself.

And honestly: How many people in the US would actually change party only because of irresponsible spending? In my view, this is like priority number 4 at best for most people, and many at most moan about it instead of letting it drive their political choices. Political representatives also basically only ever throw it around as accusation against opponents, without ever actually doing better themselves (just consider the Trump-1 promise of getting rid of the debt during his first term: not even an attempt was made).