People use the other options of joining the army, apart from that there is marriage to an American citizen, green card lottery and being in a highly skilled job. The lottery also has caps on citizens from populous countries like India,China, Nigeria etc. As a Canadian citizen it is easier to enter the country , other citizens of countries in Asia and Africa have an even harder time just entering the country. Once people enter the country they usually find ways to turn that into citizenship. On a student visa you have work restrictions (only on campus employment) and once you graduate you can apply for 1 year special type of visa called an OPT if you are a STEM major you get the 1 year plus a 17 month extension provided you still are employed. It is easier being a Mexican illegal and apply for amnesty than the normal routes which take years. I think it is easier for politicians to ignore immigrants who come to wash dishes and do low skilled labor than those that compete with the middle class.