Questioners also probably aren't as interested in the MozPoint system as they are with just getting an answer.
Additionally, inserting yourself into conversations online can be somewhat intimidating to the uninitiated, and it's possible they choose to maintain as low a profile as possible. Thus, they don't post unless absolutely necessary, and refrain from using the "thumbs up/down" or "best answer" post modifiers.
Just my guess.
Edit: Also, if only the person asking the question can mark an answer as "good," what qualifies them to do so? They were the ones with the question to begin with, so unless they act on some advice, they can't be sure whether or not it will work. So, say they act on the advice and it does work... well, they probably aren't as concerned with coming back to SEOmoz just to give some random person +4 points.