Why do Mexicans call Americans “gringos”?

I beleive some Mexicans heard the song "Green goes the mountain" and the understood that as "Gringo" (at least the Green Goes part).

Gringo (feminine, gringa) is a term in the Spanish and Portuguese languages used in some countries of Latin America to refer to foreigners from different cultures especially from the United States, but also from Canada, the United Kingdom, Australia and elsewhere, including in some cases other countries of Latin America itself.

I have wondered this myself. And also why do they call their children mama and papa? I have always been puzzled by that. To call a 2 yr old mama or papa? Can you explain that one?

okay thats too much right now cuz i am at taco bell right now ordering and i lost my train of thought cuz i was thinking about this one hard its a hard question vanna can i buy a vowel please i want an A