From the point of view of someone who has worked in loads of restaurants, you just don't sell enough food to pay the staff you need to keep on (dishwashers, cooks, extra waitresses/busboys depending). And it takes FOREVER to clean and prep a kitchen, so there are usually people back there till 1am anyway--getting ready for the next day! Especially if you're a semi-high-end pub where people expect a certain level of food (as opposed to a take-away). We used to keep a "bar"menu that was basically nachos as the waitresses could make that themselves, but nothing else. There might also be legal implications in some places, although I'm not sure about that. (licensing and etc).