Because all those plants grow in low light. Not to mention if your tank has been around a while you probably don't need root tabs because all the poop and uneatin food and stuff gets used as nutrition for the root feeders. Most root feeders will grow just fine without tab, but say they grow about a leaf a week, if you add root tabs they might do 3-4 a week.
High light with CO2 is typically for nicely colored stem plants, the reds and purples that are hard to grow.
Adding nutrients to the water column is only useful in those plants that take it from the water column like stem plants.
It is important to know how your plants feed. My wifes step Mom was having a problem with algae in her tank where she had high light, CO2, and ferts. Yet all the plants were swords and crypts all root feeders. I told her to stop doing ferts because none of those plants are likely taking in any of it. She did and her algae problem started going away.
Don't just throw stuff at your tank unless it needs it.
My low light tank I don't have CO2 but I use root tabs and nutrients, but I have ludwigia repens and rotala indica so I keep the ferts up for them and I use root tabs in all the crypts and vals. But they have grown fine without them in the past.
