CO and CO2 are different. Like you mentioned, CO is bad for us because of our hemoglobin. Otherwise I could imagine life using an otherwise chemically interest

CO and CO2 are different. Like you mentioned, CO is bad for us because of our hemoglobin. Otherwise I could imagine life using an otherwise chemically interesting molecule like CO (it can oxide further). CO2... not so interesting. Aside from the reaction to make CaCO (?) and acidic when dissolved in water it’s quite limited being fully oxidized. You need the ability to make things out of your molecules. Stuff at the ground state are boring! Plants expand energy (otherwise wasted sunlight) to convert CO2 into O2 to make an interesting molecules, namely starch with which they can do something. If they had an endless supply of starches, they wouldn’t have bothered evolving with photosynthesis - just like we don’t care for photosynthesis since we have plants.