Good fences make good neighbors, and walls make better ones
I won't be the first to observe that "something there is that doesn't love a wall." Robert Frost's well-known poem has been cited here, and here, and probably other places, in discussions of whether a wall is useful or desirable. To Frost's fan list, we can add leftists.
For those unfamiliar with Frost's poem and without time to go read it at the included link, it describes a wall-mending session Frost has with a neighbor as they rebuild damaged sections of the wall between their properties. The neighbor believes that "good fences make good neighbors," whereas Frost wants to know what is being walled in or out and who might be offended thereby. Frost recognizes the utility of walls as separators for protection (he cites cows as a problem) and, for reasons I've never understood, worries about someone taking offense.
What could be offensive about a wall, which is as passive a defense system as one can imagine? If you are worried about offending others, you are already on a slippery slope wherein you can be manipulated by anyone willing to claim hurt feelings. If Mexico chose to build a wall, I wouldn't be offended. Amused, perhaps, but I'd cheer them on.
Remarkably, Mexico prefers to let its "cows" wander, letting them feast off our resources. If only they were literally cows! They wouldn't ever rape us or kill us in automobile accidents. They would never sell us drugs. While they can tunnel under walls or climb over them, these are not reasons to refuse to build a wall.
Vienna in 1529 had to repel an army under Suleiman, and part of the defense was the walls around the city. The Ottoman army had sappers who tried to bring the walls down, but the Viennese were able to defeat the attempts. If walls were truly useless, would Suleiman have wasted time trying to destroy them?
In the book of Nehemiah in the Bible, the wall around Jerusalem was the first thing Nehemiah chose to rebuild. Even though the rebuilt wall wasn't as structurally sound as it originally was, the "neighbors" complained to Artaxerxes, king of Persia. Even a substandard wall is worrisome to those who wish you harm. Does that sound as though a wall is ineffective?
As far as climbing over them, a 50-foot wall needs more than a 51-foot ladder, since the ladder has to rest at an angle on the wall. The more important point is that there isn't always a 50-plus-foot ladder handy. A fence is a less substantial defense artifact and might be relatively easy to cut through or bash holes in, but a wall takes time to breach. Yes, explosives could take down a section of wall, but illegal aliens are sneaking in, and explosives are noisy. Given enough Border Patrol resources, officers could be on the spot in minutes after an explosion is detected. Even more important, most illegal aliens aren't going to be carrying explosives any more than they are ladders.
There are places in Mexico – e.g., Ciudad Juárez, across the border from El Paso – where someone standing on one side of the border can shoot across the imaginary line to the other side. There have been reports documenting this capability. A fence wouldn't stop a bullet much more than the non-barrier of air does.
Good fences make good neighbors because they limit the amount of harm your neighbor can do to your property and claim that it was unintentional and ask forgiveness. Good walls make even better neighbors, but only if they are built and maintained, and backed up with other means of defense.