Comment by ipaddr

They pay for the cost of that service. This is an optional service for those who choose to register a vehicle. Charging people things based on yearly income means the richest people will pay nothing as the bulk of their income isn't realized yearly. Maybe you want to charge people by total wealth but in those cases the richest have wealth available under different names in parts of the world unauditable by a state agency. Add in the cost to determine these figures now it costs a few thousand to register a vehicle.