Co2 is used as a marker of when the air is too stale inside. Why not just mass produce real sensors(most <$50 sensors are currently are fake and just guess bas

Co2 is used as a marker of when the air is too stale inside. Why not just mass produce real sensors(most <$50 sensors are currently are fake and just guess based on VOC) so we can make those cheap, since it's already something we should probably keep track of? Maybe they could be integrated into phones.