Mostly because CO2 has a high operating cost.
The main thing is balance. If you're weak in some area -temps, humidity, lighting, ventilation, etc - you improve that first, but going big won't help unless you improve the others too. I went full-max on my lighting, and my grow actually suffered.
The foliage was over-stimulated for the environment. I had temps under control, but not humidity. You need humidity when you're running sun-scale lighting.
The entire pace of the grow was changed, but my environment was only as good as the atmosphere I could keep. I should know better. A fuchcia simply won't survive in a windy enviromment for instance - I've tried repeatedly. You can hang it in a nice corner with dappled shade on a lakeshore - seems like it would work - but nope, the constant wind off the lake will dessicate it. Lotsa light in a drier than optimum environment will just make things worse.
Ventilation directly screws with temps and humidity. High ventilation makes RH difficult to control, but without it temps get too high.
It's about the balance. Every factor has to keep pace with the others.
As far as light - 50 watts per square foot of canopy is still a good rough measure of "optimum light". It's actually 20-25 PAR watts, and varies from source to source but not by a ton. If you have that much, you need to look to the other factors before you go bigger.
Balance.