Exactly!
There’s no perfect anything in this old world. If you read the statements by the officials in charge of border security, what they say isn’t that the new wall sections are a perfect solution but that it’s a heck of a lot better than what was there before. And this is with only a fraction of the wall in place and without the detection/monitoring systems being fully implemented.
add a second connection across them... problem solved
I expect if they implant various solar powered detectors on the US side that transmit, and are shielded from someone with a gun taking it out from the Mexican side, and they detect noise and vibration, even something like that would do it.