They aren't just coming from Mexico. The guberment is estimating that there are 8-13 million illegals here. Yet, there were an estimated 650K illegals who graduated from high school last year. I'll betchya that there are a lot more illegals in each of grades K5-11 then there were in grade 12. And....how many don't go to school.
Do a google search on alien smuggling. You'll find that Chinese will pay up to $50K to get smuggled here. Now, that's a lot of money for a poor dirt farmer. They're being smuggled in as indentured servants to pay off that smuggling debt.
The rate for a coyote to smuggle middle eastern types across the border is very high. So high, a coyote could probably retire in southern Mexico on the fee.
BBC had an article a few years ago that human smuggling surpassed drug smuggling in profit. Heck, alien smuggling will only get you a slap on the wrist if caught.