It's the same reason our leadership stands down at the borders applauding as a conga-line of millions dances in illegally...During war time no less.
The same reason our government stood by and watched the entire economy all but slide off a cliff into an abyss of greed and corruption while enjoying their fat salaries, top shelf benefits, while planning garden parties in their exclusive high end gated communities.
Then the answer would be...because the American people allow it.