You're getting 3-5% cash back because they charge the merchants 3-5% extra on your purchases. That's it.
EDIT: point was, it's really nothing to do with whether other people are carrying balances or not.
EDIT: point was, it's really nothing to do with whether other people are carrying balances or not.
0: https://paymentdepot.com/blog/average-credit-card-processing...