Let's say you owe me $20.
You decide that to pay me back, you'll set up a lemonade stand. There's about $50 in overhead: lemons, sugar. And don't forget wages for the younger siblings you'll be pressing into service to man the booth.
In the meantime, I decide to charge you $13 in interest. And suddenly, you find yourself needing to raise $100 to pay me that original $20.