Why do you have to use a credit card to rent a car instead of using cash?

This is so any damages or problems with the car can be charged to you. Oftentimes a rental car company will allow you to simply drop off the car in the lot and won't inspect the car for damages until long after you are gone. With a credit card number, if the seats are torn up, there is tobacco smoke odor or the car is dinged or damaged, the rental car company can charge those repair and cleaning fees to you. If you refuse to pay, the rental car company will still get paid by the credit card company.