Because no one is going to notice the noise and shower of sparks given off as they slowly, VERY slowly, grind through the barrier… oh, wait, yes they will.
The wall doesn’t have to be impenetrable, and there’s no real economically viable way to do so, rather the wall only needs to act as a visual deterrent and as a means of significantly slowing down illegal immigration.
To actual organized criminal operations… human trafficking and so forth… yeah we need MORE than a wall to deal with that issue.
The wall is just ONE PART of a greater whole designed to thwart illegal immigration.
Why do you want people illegally invading this country?