I don't know anything about websites. I had ebooks in mind.

(anecdotally) PDFs usually come from many people, departments, companies, and apps. It's hard to shoehorn in accessibility if someone didn't add it in at the origin (like in indesign or whatever app they used). Or if they printed to PDF, whatever accessibility they had would probably be lost. Much of the time it's like working with a raster image with some embedded text. Not really the same as being able to edit a proper semantic document.

With a website and available source code, any dev working on it later on can still add accessibility, tweak contrasts and fonts and add screen reader hints, etc.

It's much harder to do so for PDFs after the fact. And PDF viewer apps may or may not even support the accessibility annotations. By contrast all the major browsers and operating systems have OK support for web accessibility.