For my family and me it's not about calories, it's about nutrition. Vegetables are an excellent source of all kinds of vitamins and minerals, especially ones grown in quality soil with quality seeds. It's also about fiber. We get our calories from fats and protein.
In general most people planting these gardens "ad-hoc" and not doing it more or less "as a job" probably don't have that much dedicated surface for this, nor can put in that much effort/time into it in order to obtain more than a nice and tasty complement for their meals. If you can consistently offset a shortage for your family then you're pretty much a farmer.
And I say this as someone who actually cultivated such a garden for years, with help from the family. I allocated a lot of time each day to tending that garden, more than most people would be able to do these days and have a stable job, even with WFH.
It was just a pretty hard but pleasant activity, relaxing, I got fresh air and exercise, and I could get some really tasty vegetables but nothing that could "feed the family". And my livelihood didn't depend on the success. Like a dessert at the end of the meal: great but not quite main course.
And it incentivizes good composting and soil practices everywhere, which help all of us choose better choices in what we support for businesses and governments.
