Because the simple fact of the matter is that if people did care, what you're describing is exactly what would happen. HP (or someone else) would do something like this, a competitor would start buying ads talking about their open source ink cartridges and HP would start losing market share.
BTW whenever these "how are you supposed to" things are pointed out, people are quick to say "just use ..." with several conflicting recommendations, which actually doesn't counter but support the point that finding out isn't easy.
I find this is why I am increasingly outsourcing my purchasing decisions to Costco. It may not be the best option, but I typically only have to pick from ~3 models, and it was good enough for them to stock it on the shelves. More skin in the game than just a digital listing.
In the UK here I remember it being a question on the panel quiz show QI, about being the most expensive liquid on earth.
Companies exist only for themselves and for making money, and not for serving the people or making the communities they operate in better. They will happily hurt people and destroy the environment to get even just a little bit more wealth.
We grant them the privilege of existing as legal entities though, so maybe we should start demanding they start doing more for us in order to keep or attain that privilege.
I don't know what the actual numbers are, but if I had to guess I'd say Brother has been stealing market share from HP for a while now, because their products are better. It's one of the top comments practically every time there's a thread like this.
