Sure, but that is then telling other renters that they are not considered good enough.
It's a difficult exercise in politics, don't you think?
To do this properly, every renter would need regular check flights with an instructor. Somebody has to pay for that, whereas under the "usual" system if a renter flies regularly enough no instructor is ever needed for him. But he might still be a totally crazy pilot, bending the plane every time he lands.