Why do Mexican Immigrants & some Mexican citizens believe lies about history?

If illegal immigrants could read, they would believe differently. They are brainwashed by La Raza, the Mexican government, and Aztlan proponents into thinking that the U.S. stole the land of their forefathers. Very few of these people had ancestors living on this land considering in 1848 only 100,000 Mexicans lived in territory now owned by the U.S including 60,000 in what is now New Mexico. The maximum time they could have possibly lived on this land as Mexicans was 27 years. They were Spanish before 1821. The territory was impossible for Mexico to govern due to terrain, distance from DF, and a rowdy tered population.