A big part of why restaurants cost so much is because there's a wide range of things that are cooked to order in single portions. If there were larger batches cooked, a more limited number of options at one time, less meat and other expensive ingredients, and it was largely self-serve, the cost could be cut by well over half. As a bonus, it would remove much of the choice anxiety in menus.

I can't speak for all-you-can-eat buffets but the efficiency aspect is a large part of what makes college cafeterias viable, although those are basically the same thing.