It's just... Many (most?) of them seemed sincere. Neville Goddard, for example, never accepted any payment, so he wasn't trying to scam people. And many (most?) of them seemed to profess a belief that there is no need to get old, sick, or die - that as long as you keep your thoughts focused on a healthy, vibrant image of yourself, you will stay healthy and vibrant. And yet, they all died, most at about 70.
I'm actually hoping there is a good reason other than "they were misguided" ... Whenever I read one of these books by a well-respected new thought person, I google to see what happened to them.... And I haven't come across a single one who lived to say 90 or 100 (much less forever), and they all appeared to get older just like the people who didn't share their beliefs... None seem to die of anything nasty, like cancer - but then sometimes it just doesn't say what they died of, so who knows?