It's miles, upon miles of wall through seismic areas, not a random prison. A prison is much more structurally sound then a wall traversing the landscape. The wall travels the landscape, and every shift of the earth there will effect it. You see what happens with roads, and the wall is less flexible to such things. The upkeep would put the cost of putting it in to shame. On top of that I stand by how easy a wall is to climb. People aren't ignorant enough to let walls contain them, like many animals are. Let's face it, that even the republicans with all the control, aren't too keen on this. They had all the time, but they couldn't do that, or repeal Obamacare. Why? Because the numbers didn't favor them doing it, just to appease the Trump crowd. Obamacare is becoming More popular then ever, and a wall that wasn't favorable to begin with is less so now.