As a Mexican living in Mexico, I really like chipotle, I eat one every time I'm in the US, they are the greatest mix of American and Mexican food, still I don't understand why there aren't authentic "Taquerias" (taco shops) in the US, I haven't met a foreigner that doesn't love tacos from our taquerias that you can find in every corner in Mexico City. The closest I ever tried was at a Taco Truck in LA, but that was just one food truck.
There are taquerias on virtually every street corner in Houston.
Go deeper into the US, though, and you're right; they become increasingly rare. You can still find gems of authentic Mexican food in almost all sufficiently large American cities, but you have to specifically seek it out in many cases.
You are maybe right, when I was in LA I wasn't actively searching for taquerias, although I ate at couple of them , like I said the most authentic tacos were the ones from a taco truck.
