One of the reasons I have such a hard time agreeing with the "but they're illegal" argument is that I know that the system of legal immigration is completely, utterly, hopelessly insane. It was apparently designed 80 years ago to shut down the open immigration that existed prior to that point, and it does a good job. Or it would if it could possibly be enforced.
A few items: legal immigration from Mexico is limited to about 26,000 people a year, not counting spouses or minor children of US citizens. Of these 26,000, most of the slots are reserved for other relatives. There is almost no chance any given Mexican can legally immigrate unless they are related to a legal resident. The quota for Mexicans is the same as for Somalis and Nepalese. In short, at present, there is no general immigration to the United States, and what little there is is apportioned without regard to things like geography, need or interest.
We need to change these laws, and drastically. Between the Civil War and the 1920's, immigration was open to nearly all who came. My ancestors, probably your ancestors. Then they closed the game down. It's beyond time we reopened it, and the waves of illegal immigration are the wake-up call. The system is untenable and has been for decades. No wonder they jump the line -- for most of them the line doesn't move. It's not about amnesty, it's really not about legality. It's about whether or not we will allow meaningful immigration again.
Instead of asking why the dog peed on the hydrant, we should be asking why we insist on standing between the hydrant and the dog.