Let's take your logic one step further:

Why don't we just tear down all the prison walls?
What stops prisoners from digging a hole under the wall?
What stops prisoners from blowing holes in the wall?
What stops prisoners from buying a ladder to climb over the wall?

Absolutely nothing will stop truly determined people. Nothing.

What the wall will do however, is deeply effect local animal's migration routes, and cause some already dwindling populations to die off. It will be an environmental disaster.

Then - there is all that money, that SO is needed elsewhere, and could do so much real good ...

And it will be made with magic fairy dust so that anyone trying to go through/over/under it will instantly be transformed into an ugly spider.

In his thinking prolly machine guns...

What do you expect? He's a real estate developer who thinks the only way to get somethings done/show you did something is to build something. And of course he knows it'll be called Trumps Wall, thats another selling point to him.

So thats all it'll do - make him think he did something.

A wall would be a massive waste of money and a giant scar across America showing a time in history when the American people lost their way.