It's important to remember that the modern mega-farms are only cost-effective due to massive subsidies. A typical modern farm loses something like a dollar an acre per year and would fail immediately without subsidies. They're not profitable.
While they are certainly highly efficient for a narrow range of species, they are physically incapable of producing the vast majority of food species at all.
Comparing an acre of modern farmland growing modern corn in the modern way with modern subsidies to an acre of a Permaculture or Syntropic food forest:
The modern farm will produce far more of that corn than the eco-farm. But it will require inputs: fertilizer, irrigation, and everything concerned with the farm machinery, etc., and only produce that corn. Everything else: water, land, machinery, people, gets used up or polluted by the process.
In contrast the acre of food forest might not produce any corn at all, or it may contain four or five or more species of it, as well as fruit and nut trees, bananas and plantain plants, climbing fruiting vines, berry bushes, root crops, flowers, cucurbits, bamboo, etc. resulting in dozens of different yields. It requires no inputs.
It's more labor intensive and cannot (yet) be automated. But it's fun, easy, profitable, and it produces the material needed to make more of itself, so it's scalable.
However, grains are the staple food, not just in the US, but around the world. To my knowledge no other method than having fields of grains buffed with fertilizer comes close to its caloric output.
You're saying it requires "no inputs", but it actually requires a lot of land. The inputs you describe are plentiful, land is scarce.
If soil really does get used up as some scientists predict, we might naturally shift towards these kinds of farming, but we might also figure out a way to replenish the soil.
[citation needed]
Not sure what kind of garden you are talking about, I like gardens to plant vegetables and such, but their caloric output is very low.