Why haven't they caught on?
Because the burger tastes like shit, that's why.
Okay, not actually shit. I would prefer shit. "Meat" without flavour, but apparently with added grease; something orangish that they call cheese; wilted but somehow still watery lettuce; "relish" or "ketchup" to provide what little flavour the whole thing actually has, and that flavour being most disagreeable to boot; and bread so loaded with sugar that I'm shocked it doesn't shatter like cracknel.
No, I don't like fast food burgers; I'll make my own real hamburgers thanks.
Love my own burgers. Love fast food burgers. Love all burgers. They're all different, and serve different purposes. I will have all the burgers.