It would be too expensive. It's impractical. And it wouldn't help; those determined to get past it would find a way. It's a symbol, and nothing more. I seriously doubt that even the president, as disconnected from reality as he often is, really thinks otherwise- or ever has.
That said, is the frenzy the Trumpists and the Know-Nothings raise when an illegal alien commits a crime really justified? If so, to what extent? How big a problem is illegal immigration., really? I mean in fact, rather than simply as a rhetorical device.
And who, really, is crossing the border illegally? This will cast some light on the facts, as opposed to the rhetoric and spin.