My question is this: Why would someone who thinks that he will be successful at doing something, wouldn't do it?

My guess is that people who don't put their thoughts into "action" are the people who think that they won't be successful at it. Maybe they're imagining mostly negative outcomes.

Maybe people with more imagination can imagine more negative outcomes. If you don't have a lot of imagination, then you might not care very much.

So you can easily "test" this: Try imagining mostly positive outcomes. Would that make you more likely to put your thoughts into action? If you're hesitant, do you notice yourself imagining negative outcomes?