Yeah, I think the point about editing is a very good one. There is something comforting about them and perhaps that's it (+ maybe we are used to them being A4 pages, so you know what to expect). I think also the lack of flexibility with rendering is good, if you see it on one device you know exactly how it will look on another device.
But you can actually edit PDFs if you're into pain. For immutability there are hashes, signatures and so on.
I've seen rendering differences on different readers over the years. Rarely, but it happens. Probably not for basic documents or scanned papers. At least with HTML or Markdown you can read the source.