Mexico, even in 1848 when it got its ass kicked by the USA and subsequently sold land to the US, was an artificial creation of SPAIN, a European country full of white people, and later controlled by France and Germany, also European countries controlled by white people.

Before the Spaniards arrived, there was the Aztec empire, but it did not go all the way into the land now known as the southwest USA. It didn’t even go to the modern-day border of Mexico. It centered around modern-day Mexico city.
The people that DID live in what is now

the southwest USA, were various indian tribes who were either conquered by the Spaniards, or managed to live their lives unmolested. Guess what happened to those indian tribes once the USA bought the land – they immediately became AMERICANS. None of them got booted to Mexico.

Nobody that lived in those territories got booted off their land into Mexico (there may have been skirmishes later with indian tribes, but there are no modern-day Mexicans descended from these tribes – they all remained in the USA).
So, the end result is that modern-day Mexicans

are claiming the right to land that was never theirs before Europeans came, land their ancestors never lived on, and would never have even known existed if the Europeans hadn’t gone a-conquering. They’d still be running around Mexico City cutting each other’s heads off to

increase fertility and bring in a good crop.
Meanwhile, the ones who converted into Americans on the day the treaty of Guadalupe-Hidalgo, now have descendants who are glad to be American and probably don’t want all of these illiterate Mexicans pouring over the border.