Without interest, why would anyone lend?
Interest is a a fee paid for the risk that you won't pay back other peoples money. If it were your money why would you have to borrow it? While it might not be the bank's money, they are acting as an intermediary for the depositor.
Now if you were talking about why a country has to borrow from a private Central Bank… you would have a point.
Expand YOUR THINKING or be more precise.