I guess cooking for many people at once should be more efficient in principle.

Consider you're running a restaurant that serves warm food from 10am to 2pm. One cook prepares 100 warm dishes of 2 or 3 different categories in advance, by using big kettles and a large amount of ingredients.

It's more efficient to buy many ingredients at once, then cook them in a big kettle, then serve them in a 100 plates,

instead of every one of these 100 people going grocery shopping, spending 30 - 90 minutes in the kitchen cooking, and then eating alone.