I’m sure part of it is that open-source hardware in general is “harder” to do than software, but perhaps there is something more to it… such as people generally not wanting to invest time creating a more-crappy version of a black box that already does it’s thing well enough. Perhaps it is only after printers get significantly worse or more annoying that people will start investing their own time creating alternative firmware/hardware.
Most likely the printhead is the problem. To be able to sell those at low cost, you’d have to make a lot of them. But to start a production like that would take a shitton of capital which a startup doesn’t have access to. And HP&Co. won’t sell you printheads because they don’t want competition.