One of the weirder (and wronger) bits of "folk wisdom" surrounding the PhD is that no one reads PhD dissertations. If you look online, you'll find many posts like this one, which ask why anyone writes PhD theses when no one reads them.
This would be a good question, if it were based on a true premise. In fact, if you write even a moderately decent thesis, it will probably be read by many researchers in your field. This is for a simple reason: a thesis has a much higher page limit than any of the papers upon which it is based. As a result, there's more room for the author to explain their results. Furthermore, because it is the author's second attempt at explaining the material, they will also have a better idea of how to explain their results.
When a PhD student makes a discovery or an invention, they and their collaborators will write a paper about it. Papers are targeted at the leading experts in the field, and since the leading experts (pretty much in virtue of being experts) already understand all the basics and know where the state-of-the-art is, research papers are usually diffs. We explain only the novelties of the work, leaving the shared basics of understanding unexplained and tacit.
While this may be fine for experts, it makes accessing the literature difficult for novices (or even experts in related areas). It's like joining a long-running conversation in the middle: there will be a lot of shared context that the new reader lacks.
This is what makes PhD theses so valuable. Because there is much more space, the author of a PhD thesis has the room to explain what they did, from whatever point they think makes sense, rather than solely in terms of what the delta is. So whenever I find a paper I don't understand, I start looking for the PhD thesis based on it. Nine times out of ten, the thesis is vastly more understandable: "obvious" lemmas will have explicit proofs, algorithms will have detailed pseudocode, and the right intuitions and perspectives to take about the topic will be spelled out.
So why is there so much negative folklore around it? My hunch is that it is a way for students to psyche themselves into actually submitting their dissertation. Many PhD students are perfectionists (since they are generally bright people with high standards), and they find the thought of their dissertations becoming part of the scientific record very stressful. By telling themselves no one will read it, they can overcome their perfectionism and actually submit the document once it is done.
Writing a thesis is hard work, so I don't begrudge anyone the psychological tricks they might need to actually finish. But I think anyone who has finished writing a dissertation should feel proud of their work, and accept that many people in their field will, in fact, read it. (And if you are a researcher not yet in the habit of reading theses, let me suggest that you start -- it is a superpower for understanding things!)
