That is because burritos (along with many other such "mexican" foods) are an American invention. There is no such thing as a burrito in Mexico, unless you are at an area close to the border.
*cue k_s's vomit emoticon*
What? You think there isn't any authentic Mexican food outside of Texas? You'd be surprised. And yes, I know that Tex-Mex is not Mexican food.
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