Check out how they work.

One kind emits light through a folded optic path through the test gas and detects the difference in the absorption between wavelengths of light are used. The difference in absorption is minute so the optical path needs to be as long as possible which makes it hard to make the detector small.

The other kind detects the sound emitted when a precise frequency of IR is transmitted through a gas. If the frequency of the light is just right, it will be absorbed by the CO2 causing it to heat up ever so slightly which causes a tiny bit of sound.

Both of these are incredibly ticklish devices to design and build in mass.

That you can get these for as low as about $20 in quantity is actually mind bogglingly cheap.