How or why Scott Adams went completely of the rails is perhaps something we'll sadly never understand. Was this opinions he'd always had, but suppressed, did he somehow become radicalized or was it perhaps medically induced, e.g. a stroke or something. It was incredibly sad to see him throw away his life's work and go down a path most of us at least hadn't foreseen and die having alienated his fans.
If you want an explanation for why he would try ivermectin for cancer treatment he had a lot of beliefs in that vein for a long time. I consider that tragic for him.
The Secret has sold 30 million copies.
And at the end of the day, it's prayer. 'Prayer helps, somehow' is a very common worldview.