> "ask for returns in the contract when you give the money"
This is literally what interest on a loan is... I give an amount of money and at the moment of giving I specify in the contract how much additional you'll pay me back for the value I've provided to you. Like any other service or good.
Unless you mean "returns only if the debtor succeeds"... But that completely de-risks the act of seeking a loan (for the loan seeker), which destroys the very kernel that allows proper free markets to bring value to society. It reduces _information_.
Without the assurance of payment even upon failure, there's no reason every person on earth wouldn't seek billion dollar loans for their crazy ideas, whether or not they are experienced, confident, ready with a plan, etc. You WANT the odds of their success reflected in the agreement. It's basically a spam filter.