Because reputation doesn't really do anything, and just because someone doesn't post doesn't mean that they can't accurately judge the value of a post.
It does do something in a sense. Otherwise let gamedev scrap reputation then. Reputation could roughly (note, just roughly) allow you to judge the competence (on technical subjects) of the poster. What of a new user who is very competent but has 101 points. Surely within short time they would build their rep.
And provided the bar to start voting voting up/down is not too high (probably something like 3 to 4 post or 160 rep) then it would have negligible negative effect but effectively be able to filter off accounts created for the sake of downvoting/upvoting