You may be licensed to drive, but that doesn't make you a driver. You're not a driver unless you're driving. And you can't drive (legally) unless you're licensed. Ergo, every driver is required to be insured.
So if I'm in California, and I borrow a friend's car, that's illegal? I've never seen a car insurance company will to write a policy for a non vehicle owner or for a short period of time.
Every state is different. In Michigan, you don't have to have car insurance unless you actually own a car. If my friend, who doesn't own a vehicle, drives my car, they are covered by my policy because, in Michigan, the coverage follows the vehicle. In other states, the coverage follows the driver, so every driver has to carry insurance.
If you are uninsured and plan to drive a friend's vehicle, I highly recommend your friend call their insurance company to ask if coverage applies if you are involved in an accident. If you have auto insurance, then it's still worth asking if their coverage is primary or yours.
As I've said, because every state is so different, it's best to get answers from your insurance company instead of the internet. Trust me. I've seen too many people burned (and then pissed at me) because they didn't understand their coverages and the cop or the body shop or their friend or their cousin's roommate gave them the wrong information.