Sorry.. are you saying the 'eat leafy green vegetables' thing is wrong? I understand your underlying concern is the lack of calories, but we don't eat purely for calorific content: vegetables have vitamins and fibre without which we get unhealthy.
Your 'useful' is what worries me. Please don't eat a diet absent any greens except on doctors advice.
However, growing calories is hard. It's really hard. It's just about impossible to, outside of a real, outdoor, field.
And we need to do it, to keep people fed. It's the hard part of feeding people. Urban farming of a small number of high-vitamin, low calorie crops doesn't solve a problem that anyone is likely to have.
https://www.agmrc.org/commodities-products/vegetables/lettuc...
$2 of rice (~3lb, uncooked) buys you 5,000 calories.
Lettuce is a garnish cash crop you can grow under a lamp in your garage. Rice is what will keep you alive, though.
Yes, you can grow leafy greens semi-profitably in urban environments, or in your garage.
How many calories do those leafy greens contain? Calories are what keep eight billion people alive. You may be able to grow salad garnish in your office, but good luck growing the 2,200 calories/day that the average adult needs.
There's no transition from this, to actually useful agriculture, just like how there's no transition from throwing a Frisbee, to putting a satellite in orbit.