The prevalence of processed and preserved products (both shelf-stable and frozen) is rooted in the post-WW2 economic and scientific boom. The extremely good economy meant lots of well-paying jobs and the GI Bill let returning soldiers get college educations that had previously been unheard of in their family trees. After the lean years of rationing and wartime work, people wanted something better.

The rule of the day was technology. People were looking toward the future and expecting the golden age of science fiction to be right around the corner. One of those new innovations was convenience foods that made the housewife's life very easy! Cookbooks emphasized the use of canned and frozen products that could simply be combined and cooked to create meals, like the endless casseroles. Keep in mind that this was a pre-microwave period; it wouldn't be until the 1970s that microwaves became cheap and commonly available.

This affected most restaurants as well. In practice, few people will tell the difference between canned beans and dried beans soaked overnight, especially after cooking and seasoning. However, processed meats like chicken nuggets were more the realm of late 20th century fast food restaurants that needed to produce massive amounts of food quickly and cheaply; the McNugget wasn't even introduced until 1981.

What most people don't realize is that restaurants using frozen food dates back almost 150 years! After the Civil War, technological advances were made in refrigerator cars that could ship trainloads of frozen meat across the country. Combined with the growing numbers of cattle being raised and transported to stockyards (like the famous Union Stockyards in Chicago, which produced 82% of all domestically consumed meat in the US at the turn of the 20th century), meat was now available much more cheaply and freshly than before. You no longer needed to rely on preserved meat products like sausages, salt-cured bacon, and smoked ham when tens of thousands of cows' worth of steaks were being shipped to urban butcher shops every day.

This also extended the other way with luxury goods, and some luxuries were even created accidentally by this. Lobster and oysters are expensive delicacies today, but they were once considered foods of the commoners and even the poor on the coast. It's believed that lobster had such a bad reputation because it goes bad fairly quickly after being killed so most people were basically eating them after they expired, but oysters and other shellfish were so abundant through the early 20th century that they were practically free. This changed when these items could be frozen and shipped west, allowing saloons and hotels in the desert to showcase their opulence by providing frozen shellfish and molluscs on ice. Gold prospectors who struck it rich would buy cans of turtle soup and lobster salad to eat over the fire; they may have been common elsewhere, but they may have never even had it before and now they created a market for it as a luxury good.