Because the only ones saying that are crazy crackers.

When I was a teenager in the Los Angeles area, I had a part time job washing dishes. The pay was low but it was enough to put gas in my Ford Pinto. I learned how to keep on schedule and good work ethics.
Now high school kids can't get those part time jobs.
When I was a teenager in the Los Angeles area, I could talk to my neighbors. Now I can't unless I learn to speak Spanish.
When I was a teenager, I didn't have to paint over the graffiti on my wall every week, now I do. I'm not sure what half of it means since I can't read Spanish.
On the 4th of July my neighbors proudly fly Mexican flags in front of their houses.
My aunt is a school teacher in Pasadena. In the early 90's they told her she MUST learn to speak Spanish or lose her job.
A few years ago a local principal was ambushed and assaulted by his students. The reason for the assault? "We want a Mexican principal."
The local garbage collectors had their union busted. All of the well paid American union workers were replaced with gentlemen who spoke not a single word of English. You know, to help the country.

But I will admit that they are not "destroying the country". Just some communities are being sacrificed for cheap labor.