They don't position themselves as Mexican food any more (or less) than Arby's positions itself to be authentic Texas cuisine.Because Taco Bell tries to position itself as the place to get Mexican food on the go. Americans believe that Taco Bell represents actual Mexican food, but it isn't at all. Putting a Taco Bell in Mexico, like someone else said, is like an Olive Garden in Rome. Why would somebody choose to get some crappy taco paste in a stale shell when they could likely walk 10 feet and get real Mexican food?
I'm not sure what backwater you live in, but I'd be hard pressed to find an American who believes that Taco Bell is authentic Mexican food.
It isn't any more hilarious than an Arby's in Texas or a KFC in Kentucky.
Why would anybody choose to get some crappy processed beef sandwich, when they can get actual roast beef nearby? Why would anybody choose to get a flash-fried chicken meal in a series of buckets, when they could go to just about any restaurant for an actual meal?