Sensirion makes a decent photoacoustic-based sensor called the SCD41. It's about 30 bucks in volume.

Other models are laser based (NDIR) and more expensive.

There's also eCO2 which is estimated based on VOC levels and basically useless.

So in other words, adding real CO2 would increase the price a lot. At least double it (since the sensor cost isn't the only factor obviously)

This would totally change the value proposition and target market. I guess they did their research and decided against it. There's other devices that do CO2 for around $100.