I have no doubt that many do believe this. I work with them, and I'm married to one. :)And there's a large publishing industry that feeds off of it.

In most cases I think it's a belief akin to a superstition - like walking under a ladder is bad luck. That superstition is grounded in a certain amount of reality: there may be someone on top of the ladder who accidentally drops a hammer on your head as you walk under it. So it's probably a good idea to avoid walking under a ladder. However, somehow this reasonable note of caution has mutated into a superstition, where walking under a ladder could mean you have a worse chance of winning the lottery later that day. It makes no sense.

Just try talking about death and your co-worker's parents, or something like that. You'll see a queasy look on their face, or they'll say something like, don't talk like that! Why? Because their instinctual belief is that talk about a loved one's death could lead to that death. That same logic gets fed into "positive thinking".

Like the ladder example, as you noted, positive thinking is grounded in a certain amount of reality. Positive thinking will affect your actions and the actions of others. It just has no effect beyond that.