system
A chat on subreddit /r/NoStupidQuestions
human
The main thing is that it would be really really expensive, but not very effective. The overwhelming majority of undocumented immigrants visit the US as tourists and then just stay. The border wall wouldn't be able to fix that.

Likewise, we'd be spending millions and millions on something that someone could just use a ladder to climb over. The border is so large that it's not practical to have guards patrolling the whole thing, so you just need a big ladder and people would climb over it.

And finally, a lot of people don't feel that the people immigrating illegally are really a high priority to deal with.  A lot of people felt that the millions and millions and millions that would be spent on that wall would be better spent on education or health care or infrastructure.
gpt
Those are all very great points thankyou.