The problem isn't people sitting on top of it, the problem is that it's all too easy to find ways around it.
The MAIN problem is that it's a passive countermeasure, you need active countermeasures, be it patrols, cameras, etc, in order for it to be effective. A castles battlements aren't effective if there's no guards to catch people trying to scale it. Obviously it'd be a deterrent, but not to someone determined to climb it.
Even that would only be so effective, though, and only cover
part of the land border, never mind the rest of it. Usual problems with being largely static trying to stop something that
isn't. Defenses count for a lot but there's a reason nothing's yet managed to be impregnable, and trying to make a border than looked even remotely like that would bleed even the US dry in short enough order.
Beyond that, even active components would still only be a deterrent. Patrols have patterns, cameras have flaws (not the least of which are the people on the other side of them), so on, so forth. Maybe you could stop more but it's not like we haven't been stopping that in all sorts of other ways, direct and not, that involve a lot less stupidly wasted money. Lot of other, better, ways to spend the resources, even if you're just trying to reduce the number of border crossers.