Because most are using 60,000 words to say “Take time for yourself, spend 30 minutes a day exercising, eat nutritious un-processed foods, limit your alcohol intake, get enough sleep, and be nice to people”. That’s 90% of self help in a nutshell. If you need more than this, you probably should be getting advice from a qualified professional, not a book.

Oh, and financial self help books are crap too. There is no credible advice there that you can’t get from the r/personalfinance wiki.