Why do people assume mexicans are immigrants?

Stereotype is when someone hears a racial joke or insult of an ethnic group then they automatically assume it’s true and uses it against you. I think stereotype is just weird and wrong to do because it’s basically being racist and I don’t like racist people.

One time i went to the store and saw this group of mexicans and this white guy tried asking “do you know anyone who worked in the farm? Jokingly, so the mexicans ask the white guy, “are you trying to be racist?” the white guy laughed and said “shut up” that’s when I got mad and told him, “do you have a problem with mexicans?”

The white guy just walked away and that's when the mexican group got upset and mad so they ran up to him and started to beat him up. After the beat up the white guy said “sorry” because he felt bad afterwards.

I think stereotyping is still alive because it's all over the world, stereotype or racism is even on social media so it makes people think every race or people of race is what they hear or see about them.