Because manufacturers will not mention their product is defective by design. Until I heard of the HP ink subscription scheme a year or two ago, I would not think twice if they advertised such a feature since the expectation is that I could still buy ink independently. I would not expect the printer to stop functioning if I did not subscribe.
That's not to say that I would have bought HP. I already knew that they were not upfront about a lot of things and I already had a company that I trusted. (Ironically, a company that people claimed produced cheap junk back in the 90's.) That said, there are a lot of people buying their first printer and who have little concept of what the pitfalls are. Certain types of pitfalls are rarely mentioned in reviews and, when they are, they may come off as hyperbolic. That said, even commercial products have their pitfalls. I had to deal with a Xerox printer a while back that refused to work with a toner purchased under a support contract since the printer was not covered by the support contract. (The printer was purchased separately since the Internet connection wasn't reliable at that site.)