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.