I suppose Taco Hell is OK, but with so many real taco joints around here I have to wonder why someone would even open a Taco Bell, but they do exist.
Its a bit like asking why do Olive Gardens exist in Metro New York? why buy fake when the real thing is readily available and cheaper.

I just have this thing about large chain fast food. Its always 'lowest common denominator' stuff. The decision is always how low do the have to push quality in order to get it consistent.