Money has to come from somewhere.
Just like for individuals, you can earn it or borrow it or beg for it. If you borrow it, you need to pay it back or else the lender, and all other lenders will not lend you ever again. Or only at extortionate interest rates. This is why the so called Credit Rating is very important.
I guess the Gvt. could just print more money. That leads to inflation though and everything getting more expensive. Cue for more money printing. Germany did this in the 1920s, and this led to the most MASSIVE financial crisis, where peoples money just wasnt worth anything, and prices went through the roof. The political unrest and chaos this caused helped the nazis to power.
In Argentina,not so long ago, the left wing Government decided to simply not pay back their debts. This has led to huge inflation, and what it meant for individuals was that if you had say, 10.000 in savings or pensions, it was suddenly worth only about 3000. This happened overnight, and led to a run on the banks, riots and violence and anarchy in the street and lots of people plummeting into poverty. The country is still in turmoil, and is for most people not a happy place.
This happened only a few years ago, and shows the damage of this kind of monetary policy.
You might understand it better if you think of it as if the country were a person. Say, if I borrowed money from you to pay my expenses, and did not pay back, you would not lend me again, would you? And you would tell your friends not to lend me any either, would you?! And then, how would I pay my ongoing expenses?! It is like having to pay of a minimum amunt of your credit card debt, to keep your creditors believing you will ultimately pay the money you own them.
Saying: Fuck it, and not pay would ultimately lead to YOUR bankruptcy.