Honestly, I suspect the main reason is just expense. Making a document truly and completely accessible has a lot of dimensions. From my years dealing with accessibility in elections, I recall that some major issues regarding visual impairment were font-size and contrast; and text flow, though I'm sure I'm forgetting others.
I'd think that any format that allows the reader to easily scale up the font (or even reverse to light-on-dark) would be helpful. (Similarly, simple non-italic sans-serif fonts were also preferable, I think?) I know several ebook formats can do that sort of thing, but complex image-heavy PDFs less so.
The other big aspect is text flow. The idea is to have a format in which the text can be fed as-is into a reader app and produce a clean, intelligible audio output. This is really tough to do for docs with complex layouts like a monster manual which has images, tables, sidebars, etc; as well as back-end gobbledegook that can confuse a reader apps. Also, images and diagrams themselves ideally should have some sort of meta-description so a visually impaired reader can experience it in "audio" form. Analogous considerations need to be made for tables and charts. Obviously, the more of these non-text and not-purely-text elements you have, the bigger a project those descriptions become.
In the end, I suspect doing it "right" boils down to just generating a completely different document of the same content, but formatted specifically with accessibility in mind. But how to do that in an efficient way, I'm not so sure. I'd assume that popular doc software like InDesign would have functionality to assist in that sort of thing, but maybe not? I don't know. Hopefully someone else here has some more applicable experience!
I wish had a more solid know-how, since I know this sort of effort is really very much appreciated by people who benefit from it. If you're interested in approaches, maybe a look at
web-accessibility best practices could a good place to start? That's specific to websites, but you might find a fair bit of overlap with published documents.
** Oh, and for me personally, as someone with aging eyesight, I like do appreciate options with no background images and clear, upright fonts. While I understand the utility of italics, I find them really difficult to read since they tend to be not only "curly" but also "thin". It's even worse with unusual typefaces that emulate "handwritten" or gothic or other specialized styles. I've seen more than a few handouts in publications that are almost completely illegible to me-- and I only need reading glasses! I can't imagine what it's like for someone with more serious visual impairments.