Yes, if you read the requirements and fine print, you will see that most car-rental companies requir

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I've never heard of a rental car company requiring a minimum license time. My license (NY state) shows the date my license was issued - it is 1mo ago when I moved to a new address, not 30+ years ago when I first got my license, so the rental car co will have no idea when I first got my license.
Yes, if you read the requirements and fine print, you will see that most car-rental companies require you to have had a license for a year; those that allow recently licensed renters charge considerably more for the rental.
Most travelers from Europe do better by booking the rental through a wholesaler there. The rental quote will generally include all the required insurance (you generally have to buy a liability plan to drive a rental car in California ... the only state in the US that doesn't require car-rental companies to carry their own minimal insurance plans).
I agree with Tom re SF - you don't want or need a car while in the city of SF, but for most of the rest of CA having a car is desired. Perhaps not 100% necessary, but the flexibility of having a car lets you go when and where you want.
"Most of the rental companies do not want to rent a car for more than 30 days at a time even to US citizens". Definitely not true. Do a search of any rental car co you wish and you'll find most if not all offer long term rentals.
cheap airline tickets to frankfurt germany "Sometimes citizenship is important for certain rentals" - I am not an American citizen and have never been ask for proof of my citizenship when renting a car in USA. If this has happened to you perhaps you can expand on this.