They didn’t really have a choice. The design of CO2 lasers and the size of the tube needed pretty much means that they require a dedicated machine. They aren’t suited for use with a cnc or 3d printer frame. All of the 3d printer/cnc add-on/upgrade lasers are diode based.
If you really want/need a co2 laser do a search for K40 laser. Omtech has some lower priced machines. They have their pluses and minuses and need some tweaks and calibrations to really get working at their full potential.
-S