Because the point of reading personal development books is not to be a better person afterwards. The point is to thrill in the imagining, while you are reading the book, of the better person you could become. It’s not the outcome, but the act of reading that is the balm.
I still get suckered every so often. Once I buy another, all the familiar tropes appear ... I get disgusted ... and take it down to the local library to donate it.