Why? Lending is just giving people money they value more now than you do, so much so they agree to pay back more later. The utility at the current time of that money is better than the utility later for the lendee, and the opposite is true for the lender (they will happily forego some money available now for more money available later).

If I take a loan out to buy a car which lets me find a better job because I can travel for efficiently, and I get a better job that pays more which allows me to much more money and paying back that loan is itself even easier, is that transaction zero-sum when examined over time?