Because the national debt will never be paid off.
This is akin to saying that because anthropogenic global warming is going to be a problem no matter what, we shouldn't do anything.
It's a game-theory problem, like the tragedy of the commons or Prisoners' Dilemma.
If you need collective action to fix something, and the other actors absolutely will not participate in fixing it, instead bettering their own position, you're left with the choice of doing the right-but-useless thing, and suffering for it, or doing what everyone else does.
Republicans will never do anything good regarding the deficit. So as long as Republicans are allowed to participate in American governance, no one can actually fix it. Now, I personally think the only way out is to stop playing games with Republicans, but there's no practical or moral way to do that so long as a sizable minority of the country is willing to play with them. So we just keep stumbling on until something breaks unfixably, and then a great many people die.
