You ask
‘Why not call up the Texas National Guard to enforce the border which they can do during emergencies within the state.”
A Gov can call the NG to defend state borders but can’t call the NG to defend a national border which is what the southern border is.
State Active Duty (SAD)
The Governor can activate National Guard personnel to “State Active Duty” in response to natural or man made
disasters or Homeland Defense missions. SAD is based on State statue and policy as well as State funds.
Soldiers and Airmen remain under the command and control of the Governor. A key aspect of this duty status is that the Posse Comitatus Act does not apply, giving National Guardsmen the ability to act in a law enforcement capacity within their home state or adjacent state if granted by that state’s Governor.
I would think Homeland Defense covers his ability and also for law enforcement within their home state.
Title 32 Full Time National Guard Duty can be called upon by the Governor with the approval of the POTUS.