and they have portable ones, I own one. You also need a power supply to run it and they do have portable generators that can power one. That said they aren't cheap and they aren't that easy to carry around. But the wall itself is just part of an overall barrier that includes human patrols both on ground and in air, drones, cameras, motion sensors, etc.
A barrier can be breached but the purpose is to make it time consuming to do so.