Most still come in legally then overstay their visas.There are still too many ways to get across. Many come in at border points and bribe the guards. They tried stopping and searching all the vehicles but it caused such a long pile up at the border they dropped it.
In part, this is mainly an enforcement issue. Guess who is constantly cutting taxes and defunding the Federal government? Ironic, eh? LOL
Best to have Immigration Reform, but, again, the Republicans don't want it because too many major Republican donors, especially corporations, prefer the status quo.
"The Wall" is a distraction. Smoke and Mirrors to make it look like the Republicans are trying to fix a problem without doing anything to actually fix it. The poorly educated Trumpers swallow it hook, line and sinker.