Because in our monetary system, money is essentially printed and loaned into existence without any backing other than government and consumer debt. There's no gold behind it or any other asset. It's simply paper, and the Federal Reserve has the power to print up as much of it as it likes.
When you go to a bank for a loan, the bank doesn't take the money out of somebody else's account. It merely loans it into existence through exponential fractional reserve banking. It creates loan money on the spot. You sign on the dotted line and the bank writes you a check or deposits digital credit into your account. You then have to repay the loan plus interest with the exchange of your labor at a job.
Stop and think about that. You have to work and exchange your labor for money that a bank has the privilege to print into existence. On a $10,000 dollar loan at a fixed 10% interest rate, you pay the bank $1,000 (money which you had to work for) on a loan that was created out of thin air! If you fail to pay the money back, the bank takes your collateral (let's say a car).
Most people don't understand the implications of this system, but in essence, the bank has the power to manufacture money, loan it out, and then collect the interest on something that was printed up.