Economics

Banks don't really create it out of thin air. Banks are letting you borrow money they otherwise hold, meaning they can't lend indefinitely. This is the exact same way say a library works, they can only lend you books they have, and while you are using that book they cannot lend it to others. Paying back the principal on your loan doesn't make the money disappear, it's you giving back the amount you borrowed, plus extra to compensate the bank for lending you the money. This all matters a bit more in non-fiat currencies since those are backed by something intrinsically valued (such as gold), but even in fiat currencies banks that lend more money than they have access too are over extended and generally need to borrow themselves from either other institutions or the Fed in the US. This is why the Fed interest rate impacts loan rates.