Because a lot of them are pure nonsense that can do more harm than good. There's a weird mish-mash of useful and garbage, and it's hard to tell which is which unless you already know about the field.
Some of them lack science. Worse, some of them are actively anti-science.
I agree that there some great books, with useful helpful life-changing advice.
And even the good books have maybe an essay or three blog posts worth of actual information stretched out over a few hundred pages for salability.