Because they are not bound by the same obligations to pay them back, and they are not bound by the same restrictions under which they are eligible to take out a loan. Sure, I can get a mortgage. But I can't get new mortgages every year while sitting on my old mortgages and not paying them back.
The government always pays back its loans, on time. Often it does that by taking out new loans, but said new loans are always voluntarily given by the lenders at agreed-upon terms.
Essentially it's paying your credit card bill by taking out a new credit card. Now, as you do this, you get progressively worse rates on new credit cards as lenders see you as less reliable. And yeah, if the US government goes crazy with deficit spending, that'll happen to it, too. Exactly how that happens depends entirely on the whims of lenders and their confidence in the US economy.
Isn't the constitution supposed to gaurentee basic rights? Not do stuff like demand the congress balance the budget.
It's supposed to be a check to keep states from over reaching basic rights. Not a lawbook for the federal government.
The Constitution is a lawbook for the federal government. It details the major branches, offices, how they'll be elected or appointed, what powers they have, etc.
Now, the first ten amendments to the Constitution are collectively known as the Bill of Rights, because they were intended to enumerate certain basic rights the government can't deny without due process. But amendments since then have made other changes to the government (eg changing how Senators are selected: they used to be elected by state legislatures rather than voters; or changing the dates when Presidents' terms are ended).
So no, the Constitution isn't supposed to be limited to rights enumeration.