The land we are standing on was Mexico and people from Mexico traveled freely between what is now California and Mexico. Mexico lost the war and now the land belongs to the United States. However, the reality of the people remains the same. Extended families co-exist in both countries for generations. The problem is that the normal processes of life are severely punished by current U.S. Immigration laws.
Congress passed laws several decades ago making it extremely difficult for Mexicans to obtain status. Before 1996 a person who was in the United States for 7 years and was a good person, with no criminal record, could file for and get Lawful Permanent Resident status. Then after the passage of new laws, that same person would have to have been in the United States for 10 years and have a child, spouse or parent who would suffer "extremely unusual and severe" hardship if the person were ordered removed. This has led to unbelievable and terrible consequences for thousands of Mexican families whose children were born here and go to school here.
Moreover, if someone has a U.S. born child and that child turns 21 and can petition for the parent, it will not help the parent because that parent lives in the United States without status and there is no waiver for this situation. In order to be able to file for a waiver, the person must either have a spouse or a parent who is a U.S. citizen or Resident and that person would also have to prove "extremely unusual and severe hardship".