Because the stampede when Uncle Joe invited everybody in, with benefits and government checks to boot. Realistically the border will never be sealed. There's lots of legitimate travel and commerce across the border.
If we really want to stop the illegal commerce across the border, arresting all the bankers who launder money for the cartels would be the first thing. The cartels only operate because they are making money... and cartels won't send drugs if they can't get money back:
HSBC Admit to Anti-Money Laundering and Sanctions Violations, Forfeit $1.256 Billion
$1.256 Billion has been described as a traffic ticket for what HSBC intentionally did and all the profit they made. Nobody even went to jail.
The establishment wants the illegal commerce to continue because they are profiting from it, BIG TIME... Just like all that cheap foreign labor... that's PROFIT for certain people, at our expense.