Part of what helps a player turn into a legitimate NBA player is playing time and positively reinforced development early on in their career. To some extent, if you treat a player like a scrub early in their career, that's what they're going to be.
Gerald Green´s problem is not a lack of confidence. Orien Greene isn´t out of the league because of a lack of confidence.
Stiemsma is 26 and was cut before, JJJ and Moore were drafted late in a pretty weak draft. I think the rational answer why Doc doesn´t play them is simply because they´re not ready or a lack of talent.
Certainly there are players who have plenty confidence; however, their poor development could still be attributed to other factors, like the team not doing a great job in teaching them the right way to play.
You're right, though, that some players just aren't talented enough to make it in this league. But when the team has been almost completely unable to turn any of the young players its drafted / acquired recently into rotation contributors, you have to wonder who that falls on. Other teams are able to find contributors even late in the draft.