The cost of the materials is only a small part of it. Preparing and cooking fresh food requires much, much more effort than prepared packaged foods. Microwavable or similarly low/zero-prep food products require very little labor, require only a few steps before they are served, and create almost zero items to be washed and cleaned. You would be surprised how much cheaper a flat of microwave burritos is compared to an equally caloric-dense flat of fresh or bulk goods.

Also to consider is the cost of a skilled laborer -- someone who can actually cook -- versus a minimum wage worker who only needs to unwrap a package, press a button, and plate it.