Why are mexican soccer players so nice? They're looking for the green card.
Why do you think it is funny to joke about Mexicans?
Because I'm making fun of idiots and it's not my fault if mexican soccer players are nice. They all want to come to our country and steal our jobs, so I'm fine with them being insulted.
Mexicans have been migrating to America for many generations and some Mexicans have been born there. Assuming that somebody is an illegal immigrant can be hurtful and make the person feel like they don't belong. You would not like if people assume that you didn't belong based on your race.