My longstanding theory of breakfast foods hearkens back to centuries past.
Even in the early 20th century, few Americans, for example, didn’t have anything akin to our stoves. They would’ve had a hearth or maybe a Franklin stove, which might have been banked in the winter, but would have been uncomfortable and wasteful to run overnight for most of the year. Cooking had to be minimal.
Breakfast traditions in the US probably owe more to rural traditions than urban (the majority of the US propulation was rural until the middle of the 20th century). Breakfast occupies a different place in the farmer’s life than for urbanites. Farmers were often up before dawn, and only ate breakfast after a few hours of morning: the cows were milked, eggs gathered, and the animals were fed before the farmer was. Even lunch in many countries is still traditionally a ‘cold meal’ like the “ploughman’s lunch” eaten by farmers in the field, workers in factories, etc.
Many “traditional breakfast foods,” may be traditional, but weren’t eaten hot (as we like to imagine today -but how many of us don’t bother heating our poptarts or danishes as we rush out the door?) A fire was not lit every day, and for much of the year, the cooking fire might have been outside. Food for all meals was often cooked for days in advance (e.g. breads, stews, roasts, etc.).
eggs: fresh or unrefrigerated in a waterglass (sodium silicate) solution, were considered bacterially sterile self-storing foods until a few cases of chicken ovaray salmonella were seen in the 1980s. Most are still safe - especially since cold hard boiled eggs were the most common breakfast form in he past
bacon: sugar-, salt-, smoke- and dry curing were preservative methods. Bacon can also be cooked in advance
ham: like above, but eaten raw more often than bacon
sausage: a preservation method. Today’s breakfast sausages are raw, but the old types were precooked or preserved, like deli meats or cold cuts
oatmeal, porridge, grits, etc.: often eaten hot today, but more often prepared in advance and eaten cold in the past
pancakes, griddlecakes, etc.: flour or meal, water, maybe eggs, cooked quickly; also suitable for eating cold
cereals: (muesli, granola, toasted grains) prepared in advance
fruit: I’m not going to explain this; too many puns
milk: ditoo - really really horrendous jokes being repressed here
orange juice: NOT traditional:a deliberately orchestrated campaign by Florida orange growers after WWII. Impractical without trucking/refrigeration
pizza and chinese take out: What? That’s what I prefer!
biscuits, bread, etc.: the last routine baker of fresh biscuits for breakfast, a Ms. Martha Stewart, was recently jailed for fraud, ending the practice. Seriously, these were always typically prepared in advance.
You get the idea. We now eat (or imagine that others eat) foods descended from the traditional breakfast foods of yore, but not served the same way. The “hot breakfast” was probably never the rule, even in the brief era between the modernization of the kitchen, and the two-income family.