Without getting in to the prices at all, there is something to be said for focus. The diner throws together great food as long as great is “salty and fried”, but when it comes to more complex stuff, they tend to fall down. The large menu means the time and affort to get expert at each dish is much higher, and with any turnover at all just can’t happen. Mom and pop diners can get great at their specialties, but chain diners al_ost always resort to reheating frozen product because of their large menus