SpaceEngine

Almost all the terras and oceanias with organic (even multicellular) life on them have carbon dioxide as the main component in their atmospheres. Why? That does not make sense. On Earth, plants consume carbon dioxide through photosynthesis, but on those exoplanets (even some planets that are mostly green on their lands), it seems that process doesn't work at all. To be honest, I never seen any life-supporting planet with CO₂ less than 50% in their atmospheres in SpaceEngine before. 
Why?

Actually, that's what Earth's atmosphere looked like billions of years ago. It would be nice to have some atmospheres with almost no CO2, but I guess the fact that it happened on Earth was unlikely.
https://en.wikipedia.org/wiki/Great_Oxygenation_Event

Actually, that's what Earth's atmosphere looked like billions of years ago. It would be nice to have some atmospheres with almost no CO2, but I guess the fact that it happened on Earth was unlikely.
https://en.wikipedia.org/wiki/Great_Oxygenation_Event

But even then there should mostly be nitrogen in Earth atmosphere. It's just that oxygen is scarce. 

This is because of the way the algorithm that generates the atmospheres is. Atmospheres in space engine are still a new thing and I won't be surprised if it is changed in the future. But youve also gotta keep in mind that these are alien worlds, to them Earth's atmosphere would be poisonous, not every planet with life out there will be like earth. 

The life can also rely on chemosynthesis.

Maybe there's forms of photosynthesis that are anaerobic, meaning that they don't produce oxygen in this case. There's probably plenty of other reactions that can pull energy from light other than the oxygen-releasing ones we're used to.