Universal healthcare works in various countries because everyone is forced to participate. Everyone pays into the system, everyone gets care when they get sick or are injured.

A US state can't do this on it's own because they can't force everyone to participate and they can't reject entrants from outside. In the Virginia/Maryland example Maryland would face an exodus of healthy and productive people who want lower taxes and an influx of sick people who want the free health care. It's not about "remaining equal in all other categories." It's that it wouldn't really work at all.

That's why you can't really have a system with massively different social services at the state level: the unbridled state to state migration.