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.
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.