when a illegal immigrant marries a u.s. citizen he or she becomes a u.s. citizen and does not have to return to his or her country
I don't think that it's entirely true. If you don't have any papers, then you can not become a US citizen UNLESS you have a visa or greencard.
Yes, especially for international flights. If you are just traveling in the U.S. then you only need a drivers license. But you will not make it through customs without a passport and if you need a visa in another country you need a U.S. passport.
Some people immigrate to North America looking for a better life, especially more freedom.
It is one of many factors. Still, parents want the best for their children and a good education is one of them.