...nothing else? Why is that? Please explain.

Thats pretty well standard across America doing that.
IN WV they want 2 proofs of residence 1. Phone Bill 2. Cable Bill etc. Silly but true.

It's the same everywhere. They want to make sure you have enough to cover damages (or at least the deductable.) Otherwise, you could give them back a wreck, and simply say that you have no more money to give them. You can't even max out your crd whie you have the car. They usually get a hold put on the card for the amount of the deductable. The money doesn't actually get paid to them, but they make sure the money will still be accessable to them when the car comes back.