It is the nozzle. Everything else is very simple to make because it is already done for 3d printers that are more complex than 2d printers(if you do not consider the nozzle).
5 years or so ago I made a 2d plotter with friends at my 3d printers community with the reverse engineering knowledge that we had about a specific cartridge with nozzles on it.
Printing with ink was easy, very easy. But we were interested in using it for 3d print wax, not so easy.
You need to manufacture nozzles, and that requires lots of money. That requires manufacturing plants. Very cheap in volume, but requires volume.
Open source has not volume in the millions, like big companies have, and those companies are not going to sell you the nozzles so you commoditize their professional field like linux did.