Yes but often when you go to a buffet you can get some kind of family discount no?
Think of it like this; when you buy a bus or train ticket in a city system, there is usually a single ticket, then some kind of duo/family ticket for a good discounted price. But there is never any 'entire extended family' discount ticket which costs a boat load but is still cheaper than 30 single tickets.
Just because you get a lot of something for a discounted price, doesn't make it a good deal to you personally if you only use 20% of what you have paid for