Some Word documents are like that too! Can't blame the PDF format if the source material is a bunch of scans.
html/css has a similar problem. A lot of my email is just a collection of images that contain text.
I absolutely hate those, but the HTML spec does require the alt attribute which is actually used in practice quite commonly.
I've never read the HTML spec directly until checking it now to verify your comment. I usually use MDN, which says that the alt attribute is not mandatory. [1]