Everyone thinks they want to read a PDF on something that isn't a desktop or an A4 sheet of paper, and then they try it and realize that there are basically no PDFs that are laid-out in a way that could be feasibly consumed on such a device.
In addition to that, writing heuristics to try and make it possible to unify the experience of reading any arbitrary PDF is such a quagmire/shitshow that basically no one is willing to step up and try to make it possible.