Or more precisely:

The problem with the world is fools and fanatics are so sure of themselves, and wiser people so full of doubts. -- Bertrand Russell

..Most actors don't think enough, and most thinkers don't act enough. cf. Dunning-Kruger effect.

Extroverts and Introverts typically line up with those two categories quite neatly, and in my observation tend to associate mainly with people of similar temperament (allowing them to avoid much of the pressure to be more balanced they'd find in a less homogenous social circle). I believe that this lack of balanced interaction is the real source of the problem. We need balanced pressure to both act and think competently, but the inherent discomfort makes most people unwilling to voluntarily seek it out (if they even become aware that doing so is beneficial).

I'm not sure I agree in the general case, and I think that among LessWrongers things are certainly unbalanced in the other direction.