That’s dumb, though. Why do a fence when you can do a wall with better protection?
Because it's cheaper. If the protection of a fence is "good enough" and it will be easier to get the funding then do it that way.
I haven't really studied the problem but there are probably portions of the border where a fence will be "good enough" due to the local terrain. And there are probably portions of the border where only a wall will work.
Let's get this thing built!