without trying to explain other people's thought process, I personally feel like this makes sense because both 2. and 3. are clearly true. maybe they are only clearly true for a certain type of person, in which case it's some interesting insight into the rationalist psyche.

I think this is, at least in part, historical. The Sequences are pretty self-help flavored, and I think lots of what was going on in the ecosystem was (incidentally?) piggybacking on broader self-help discourse in the early 2010s (same era that, eg, Jordan Peterson rose to prominence). LW was also part of a similar cultural sphere/wave to New Atheism, which was continuous with self-help discourse in part because the newly irreligious sometimes face adverse effects from the loss of their community and ontological framework, so prominent atheists often try to point people in directions that will help them reckon with that loss (parallel idea to Alain De Botton’s Religion 2.0 stuff). There’s also the high rates of neurodivergence. I think neurodivergent people have a higher ceiling on the benefits they can reap from self-help material, are more likely to seek it out, and are also more likely to find specific individual interventions ineffective (since most self-help is targeted at neurotypical people afaict). Finally I think self-help stuff is higher variance for most people. There seems to be a lot more variance in personal psychology than in, eg, how your muscles respond to hypertrophy or protein intake. The 80/20 of physical wellbeing is comparatively straightforward and the ‘standard advice’ is more reliably applicable to any individual’s situation. You just don’t need much literature to cover the basics, whereas reading the wrong piece of self-help can easily be neutral or net negative, which leads to a proliferation of takes. I wouldn’t be surprise if one of your other explanations also turns out true, but this may be useful background :)