It's pretty trivial for a company to make a mold for a plastic cartridge. The chips are nothing special, they are just ROMs. Cheap and widely available.
In fact, I'm pretty confident that a competent DIYer could 3D print a cartridge shell, flash a ROM, and etch a PCB in an afternoon or two.
In fact, I'm pretty confident that a competent DIYer could 3D print a cartridge shell, flash a ROM, and etch a PCB in an afternoon or two.