Transaction costs mean that people who are eating in a group don’t want to go to one store for the pizza, another store for the sandwiches, etc. The benefit from not having to make separate orders at separate stores for the sandwiches and salad outweighs the loss in efficiency from the store not completely specializing in pizzas.
The big pizza delivery chains have expanded their menus over time, from pretty-much-pizza to sandwiches, desserts, wings, salads, etc.
So it seems efficiency is not the driver.
Transaction costs mean that people who are eating in a group don’t want to go to one store for the pizza, another store for the sandwiches, etc. The benefit from not having to make separate orders at separate stores for the sandwiches and salad outweighs the loss in efficiency from the store not completely specializing in pizzas.