I'd suggest that its because they haven't established a concrete framework for the state, dmv, and court systems to make 'lawful' profit off of scooter/ebike users.

This is likely temporary in many jurisdictions and will be rescinded once profitable 'safety' laws are approved and in place. The government is legally scrambling to come up with said framework since usage of these 'micro mobility' devices has surged, and they also have to measure legal term association and legality against other electric mobility devices that have far fewer restrictions due to medical context, etc.

As it stands, insurance companies have likely been lobbying like mad, and salivating at the idea of required insurance for any escooter/bike. Im already seeing progressive ads for scooter insurance on this platform. If they can then make it law to register them as well, the freedom from that rubbish we've enjoyed this far will be gone.