There is already nearly 700 miles of fence. The hold up is Texas, and the enviros.
Texas is different than the other border states in that there is no Federal Easement, the snaking Rio Grande is the border line and private landownership up to the River.
There are many landowners who’s land runs right up to the river, ranchers use the water for cattle, farmers use the water for irrigation and some landowners have inherited the land from ancestors that date back to the 1700’s.
There is also major recreation areas + Big Bend National Park. Two very large Lakes with resorts. And there is Apache Indian land.
In the other states, there was a Federal Easement so the logistics were much simpler.
Now that said, strategic fencing is more effective, less costly and can be done rather quickly with the backup of Guard/Border Patrol and equipment.