It seems reasonable to me. The risk is primarily determined by the ability of the driver, and can be gauged by their claim history and factors like age. E.g. a

It seems reasonable to me. The risk is primarily determined by the ability of the driver, and can be gauged by their claim history and factors like age. E.g. a 65 year old female driver with no claims and a low insurance premium. If she arbitrarily allows her 18 year old male neighbour to drive her car, the insurer would be exposed to a high risk. The only way to offset that would be to raise premiums for everyone.