Are the moderators working so actively to read every post so that they award the one who post high-quality posts the merits?
I think you are confusing moderators with merit sources. Merit sources are working actively to read a lot of posts and award high quality posts with merit.
Mods have different tasks here - shortly, tthey are moderating forum. But they also can award posts with merit, like every user of this forum who have sMerit.