There's no specific regulation that doctors can't see patients in other states. Each state simply operates their own medical licensing system. You could imagine a system that works differently, but getting there would require creating new rules and resolving new conflicts, not just removing some rule that exists today.

For example, there's a certain category of hot-button procedures that California believes are medically necessary but Texas will revoke your license for performing. To set up a shared licensing board you'd have to somehow find an acceptable compromise.

You have described every single regulatory problem anyone has ever complained about in the FDA: the default state is denial, and positive action is needed to carve permitted things out of it. If this is a categorical exemption to 'the regulations are written in blood', then you're failing to describe anything identifiable about the regulations at all.

The Revolutionary War.

That's not a regulation, it's a consequence of a bunch of individual regulations.

What's the alternative? Federal licensing would be good but poking around wiki it looks like the basis of medical licensing in the first place is "states' rights" so good luck with that. Reciprocity would also be good and hey! it turns out it's being worked on : https://www.ama-assn.org/system/files/fsmb-interstate-medica... https://imlcc.com