First, they aren't Mexican. They come from failed communist nations in South America and are bussed and sent by train to our borders where they fail to qualify for asylum because they didn't apply at any country along the way.
Secondly the US has finite resources and we have a limit each year on the amount of people we take in. We are about 100x that limit this year. This means price increases and scarcity is coming like the baby formula thing.