Comment by anigbrowl

Because PDF shows you a page on screen that will look the same if you print it out, and print layouts have been optimized for reading convenience over centuries. And if you give someone with no technical expertise a pdf file, it's virtually certain that they're going to be able to open it because some kind of viewer is built into most operating systems.

You're totally right about PDF being a massive pain in the butt for any other purpose, but unless you have an alternative that handles the basic use case at least as well and other use cases way better, PDF is here to stay.