seems like, Sixt is using the German procedure to rent cars.

difficult to explain, I think...
Let's try it this way: the driving license in Germany is not an official state document. that is only your passport or ID (which doesn't really exist in US)
so, if you cannot provide that, there is not really a proof that the credit card fits to your ID

the utility bill is just a hint, that you have lived under this address. there are a couple of other countries which are asking for that, too.