If someone else has access to a vehicle, as in lives in the same house as the insured, the premiums already take into account that driver's individual driving and accident record.
If it is someone else, or several someone elses who do not live in the same household, how do you get coverage for them? Does that mean you need to arrange coverage for a person who uses the car even just once?
Do you instead require an individual to purchase their own insurance even if they don't own a vehicle, just in case they should need to borrow a vehicle belonging to another person, and if so, how could that person do so if he or she doesn't know what type of vehicle they might borrow at some point in the future?
Even with all that, under current common law the vehicle owner and by proxy the insurance provider is responsible for any shortfall involving a civil suit award arising out of the use of his or her vehicle regardless of who is driving it, whether or not the non-owner driver is insured.
State Farm's motor vehicle insurance premium revenue from at least 2011 to 2014 fell short of administrative and claims payout expenses by a fair bit. Where they did make profits on premiums was in the unregulated life and home insurance markets, and on investment returns on premiums they received and held but had not yet paid out.