Why is the SF-Class bound to the vehicle?

Hello, I bought a new car and I would like to keep my old mill and drive it every now and then. Since this would be my first second car, I have no experience in this regard and was surprised that the SF-Class is tied to the vehicle by the insurers and not to my person. I.e. the insurer does not care if I drive the old car only occasionally. On the contrary, I have to go through the lower SF-Class (always 4 instead of only 1/2 ) pay significantly more for the vehicle, because I transferred my previous SF-Class to the new car. Alternating license plate is probably no alternative, I have read… Is there good reasons on the part of the insurers, why this is handled so? Thank you!