People in the country illegally, by and large, do not get here illegally. They enter legally, and overstay their visas.

The notion that the illegal border crossings represent the main, or primary source of illegal immigrants is untrue.
It's the ones getting past all the border security legally that are the real problem (and not a majority of them, just to be clear).

If the problem that needs to be solved is the presence of illegal immigrants in the USA, then spending money on border control is pissing in the sea. The money needs to be spent on finding the illegals that are already here, not trying to stifle the trickle of desperate people sneaking across the border.