It's not diplomas it's certifications. It (e.g.) a lawyer moves between states he or she may need to take that new state's bar exam.

Ok. While it's different, the difference is subtle. As you said for lawyers there is an exam each time one changing state? Same for nurses? Engineers? I can't see good part of that.

The difference is that there is no nationwide licensing body, so the license issued by the "nursing licensing body" in Texas needs to be transferred to the "nursing licensing body" in Washington state. And they need to accept it and turn it into a Washington state nursing certification.

Because these are separate bodies there may be different rules, regulations, standards regarding what it takes to be a nurse.

Yet a doctor can travel between countries who agree to have "equivalence" between qualifications.

Law obviously makes sense when so much is state specific, I'd be less convinced by nursing.

In the case of lawyers, there are significant sets of state-specific laws and regulations, so it makes some sense to have separate "certification" by state. My understanding is that, once you have been an attorney for a certain number of years (5?), it is much easier to be admitted to the bar (certified) in a new state.

I'm guessing you're not from the US and neither am I but in my experience with things USofA it really is amazing how many ways the United States operates as a union rather than as a single country if that makes sense.

They don't even seem to try to have common laws/licensing/practices for a really huge number of quite boring matters where it would seem quite efficient and advantageous to operate the same way.