Lobbying from the AMA to limit available labour & maintain high wages

You can't even do a tiny lateral transfer in the US medical industry without reschooling, so how would it work with foreign degrees?

That's analogous to switching from being a data-base administrator to a systems ops engineer. Sure, fundamental knowledge of logic is transferable (compare: knowledge of human anatomy on a general scale, courtesy of medical school), but the intricacies and applications of that knowledge are different. Some re-schooling from one extremely specialized medical post to another makes sense.

I'm talking about, for instance: being an orthopedic surgeon in England (or even France, or Russia). I operate on human bodies. Those human bodies work the same way in the United States, do they not?

Maybe there are some discrepancies in established best practices, country to country. But there is no necessity to go through the entirety of relearning fundamental medical knowledge just to demonstrate you didn't go to a "worse" medical school. Even if you did - what should be looked at is your medical practice, not school, and your patient track record.