Despite the high level of economic integration between US states, for some reason all of them insist on having their own rules for driver licensing and car registration. When moving to a different state you are usually required by law to exchange your driving license and re-register your car within 30 days. Some states care about this issue so much, they penalize out-of-state cars that are seen parking overnight too often.
Why not simplify the system and have the federal government issue all driving licenses, car registrations and license plates? The revenue from the system could then be shared by Washington on a pro-rata basis according to the latest population census.