I feel it's because a beginner is a mostly blank slate so you can give generic advice without need to understand their personal context. Someone who is proficient will have strong context (areas they're strong in, weak in, like, dislike, etc.) so advice should be tailored to them. Writers may also not actually be cognizant of their own context so can't self select specific tailored advice books. Which is why there's historically been writing workshops, editors, proofreaders, etc.
I'm not anything like a professional writer but I was going say something like your point. There's no easy advise list for serious writers. Even more, no being able to write is specific speed bump that many people experience and getting past that is a key thing. Not knowing basic style and grammar "rules" is another common experience. After that, the rules and the choice are much less clear - the style, tone, etc of your writing relates holistically to what you want to say.
But the opposite pole is that for an advanced writers, any book you consider great is a kind of advise - it's an example of the choices one writer made to create a text said something well. If you want more here, you can also read an analysis of such texts.
And with so much available once someone is at that level, there's also not a lot of reason for people write advanced advise books.
Perhaps. It seems awfully counterproductive though, even so.
Other fields (even ones that benefit from or effectively require in-person guidance -- like mathematics, or martial arts) have books aimed at more advanced practitioners. Theoretically, some writing advice books are aimed at more advanced practitioners.
This essay was spawned by my experience reading a book that was literally intended as the textbook for an undergraduate creative writing workshop (i.e., a class for juniors and seniors who are english majors with a focus in creative writing). Surely twenty-five year olds who have already spent three years dedicated to the craft of creative writing are in the same position as their peers who have majored in mathematics, and do not need to be assigned the english equivalent of "The Complete Idiot's Guide to Calculus".
I've read a number of other writing guides, only one of which was aimed at folks who did not self-identify as writers, and they all had at least one chapter on navigating ego hangups -- even the ones called things like "Creating Engaging Character Arcs" and "Promoting Audiobooks on Kindle" that you'd expect to be highly technical and specifically aimed at professionals.
Curious how off the mark I am in my thoughts.