I do the same thing for my midfield. I play a 4-3-3 wing play tactic with a DM, and all play is focused down the sides. My MCs and DM generally end up with ratings between 6.6 and 6.8 if they don't provide an assist, score a goal, or are otherwise busier than usual. For a while I was rotating people in there to try and find someone who got higher ratings, but eventually I thought about it and realized that, due to the nature of my tactic, the central midfielders' roles are much less involved than at other teams, they're basically there to provide outlets for the winger and fullbacks as they make their way up the field, make the odd run into the box, find the odd pass, and keep the other team from coming up the middle. Naturally, evaluators will see that they play far fewer passes than other players in their position, and presumably that's why their match ratings are always ****.