Well, if there is a surplus and no debt, then the government has to find something to do with the money other than paying back the loan.
It doesn't because it's a currency issuer, not a currency user.
Imagine if you could change my bank balance just by keystrokes. If you want a $100 worth of goods or services from me, you change my bank balance by +100. If you want to tax me by $100, you change my bank balance by -100. A Jason surplus just means you've marked my balance down more than you've marked it up. A Jason deficit means the opposite. You don't actually need or want - or, arguably, even receive - my money. You want
a) the goods or services (a school, say, or a semester's teaching) and
b) to maintain the value of the units by which you mark accounts up and down i.e. their exchangeability for goods and services.
Now you could do both just by marking bank balances up and down (and everyone from Mynski to Milton Friedman has argued that you should). However, you don't own my bank and in order to change my balance you have to change my bank's balance at your bank. And your bank doesn't have money, it has
reserves, which are a kind of meta-money banks like mine use to settle up with each other "overnight" and to borrow from each other. Your bank creates these reserves out of nothing with keystrokes (you pretend it's not your bank, but everyone knows it is really). And when you mark my account up, you issue a bond, which gives someone like me or my bank the opportunity to swap reserves in what is basically a demand deposit for basically an interest-bearing savings account at your bank.
You don't have to, but it gives you a powerful and crafty tool for controlling currency users' spending and saving (tyrannical control freak that you are). You call it "borrowing" but it's really nothing like what I have to do when I need to get money from other currency users.