Imagine that Congress passes, and the president signs, legislation requiring the budget to be balanced within a decade. What's to keep Congress from ignoring that legislation and breaking its promise down the road?
A future Congress could simply pass new legislation that nullifies the law. Even if the law had some kind of enforcement mechanism, like automatic tax hikes, the mechanism could be repealed in the new legislation.
This is constitutional, and legal challenges against ignoring the legislation would likely fail.