There is already about 650 miles of fence along the U.S. southern border so what's wrong with more fence or a wall?

Obviously it will be expensive and difficult for the Trump regime to build a wall along the entire southern U.S. border with Mexico, but, when there are somewhere between 11 and 20 million people in the U.S.A. who have entered the U.S.A. illegally and when most have entered illegally along the US/Mexico border, doesn't the U.S. government have the right to do everything in its power to protect and defend its border and keep people from crossing the border into the U.S.A. illegally?

They're not trying to stop people from crossing the border legally.

But what's wrong with building a fence or even a wall to keep people from crossing the border illegally?