Why does anyone prepare their own food? I'm not an econ person, so this is a super naive question, but here goes: why, given the comparative advantage that specialization produces, does anyone prepare their own food rather than buying from restaurants all the time? It seems like there should be economies of scale with respect to food production (restaurants prepare food far more efficiently than I can do at home). Where I live, it's often cheaper to prepare your own food than ordering take out, but I can't understand why.
The weights used in the U.S. CPI (from 2016) are available here: link to BLS weights. Note that these weights are based on consumption data, so that they are meant to align with actual spending patterns.
If you look at them, you will see that the majority of the weight is either processed food or food away from home. Note that some categories explicitly state “processed,” but even things like salad dressing are put together in a factory.
Factories have a comparative advantage for processing food, but not for delivering hot food. Meanwhile, food is normally shipped in a form that can be stored. A box of pasta can be held in a store a lot longer than pasta that has been boiled and put into a container. Households do the final heating steps. As such, the premise of the question is somewhat flawed - processing is being done largely in an “efficient” fashion (although there are debates about food quality).
Restaurants are required to pay workers and rent and expect to make a profit, whereas food prepared (or just heated up) requires no payment. Cooking is labour intensive, and so it is hard to compete against \$0/hour if we look at the cost of the meal. Individuals might value their time at more than \$0/hour, but how they value their time does not matter if they cannot get more paid work and face a strict monetary budget constraint. Meanwhile, travelling to get a meal takes time. Finally, being reliant on food away from home poses risks, as seen in the pandemic.