Many folks simply watch the plant's response, not just the fish alone...........and also algae's response to change in CO2.
CO2 will have an optima for each individual tank that is different.
Yes, there is an upper range where all plants do very well and as you add more and more, this becomes apparent. But for other tanks and fish, filter systems, circulation etc...........this is often not practical due to the fish/shrimp etc.
Also, less light= less CO2 demand. This is clear in Tropica's CO2 and Light article. So if you promote LESS CO2, then promote less light, then.......you can logically suggest less ferts since demand and growth are reduced.
It's also easier to manage a tank with lower light vs high for the CO2 wiggle room alone.
But people think more light is always better.