we were having this debate at school and lets just say i pissed off a couple immagrants at our school when i said "But if you can't speak english why the hell would you come here?"
1. Often, it's the other way around. You come to the country and then learn to speak English. A lot of countries do teach English as part of everyday learning, but not all. So, some don't have the English classes available to them until after they immigrate. Not mastering the English language right away is perfectly reasonable. Like most languages, English does take time to learn. I think what counts here is the effort to learn English. Because otherwise, finding work or forming a social network can be a very difficult thing to do.
2. Where else is there for them to immigrate to? Many of the countries that you can immigrate into and have the potential to live comfortably in do have English as an officiallanguage. If not English, than the official languages are often European. So, there isn't really much variety available when you don't speak English as a first language.