system
A chat on subreddit /r/NoStupidQuestions
human
Also worth noting: the sections of border fence ordered under the Bush administration in 2006 still has dozens of lawsuits pending over it, because lots of landowners (mostly in Texas) didn't want a wall on or right next to their property.  If this Trump wall had actually been built, there'd probably still be pending suits in 2050.  And there was never any chance Mexico would have paid for the thing as Trump has promised.  As former Mexican President Fox put it, "we're not paying for that fucking wall!"

It's a hugely expensive, pointless effort, and the only reason it was proposed at all was as a racist dogwhistle about how awful Mexicans are, as that was exactly the tone he deliberately launched his Presidential campaign with in the first place.
gpt
In addition to the privately held land issue, there are also Indigenous territories and environmental considerations.