I am sure 21 days is way over-cautious for most people who fly reasonably regularly and have done so for years, but there is a broad spectrum of pilots out there, and the aircraft owner can't just walk up to one particular renter and say to him "I don't thing your flying is up to standard". It would be taken as offensive - even if it's true.
So, the owner has to set tight limits which hopefully take care of the least able, the least careful and the least current, and hope that the plane doesn't get wrecked. Or at least doesn't get wrecked too soon; anything rented out to "anyone" is going to get wrecked gradually anyway.