Even though football is a team sport, individual mistakes or poor performances can cost a team points.
In the case of Darren Fletcher, he always tries his best and sometimes even comes up with a match-winner, but on a lot of occasions his talent has simply not been good enough. He doesn't have a very good touch, his passes very often don't find their target, he's not quick be prem standards and his crosses are quite average too. So really, all he brings to the team is industry, but look at our main rivals, Chelsea and all of their midfielders are easily better than Darren Fletcher.
Also, your opening statement is not necessarily true. Fletcher outperforms his team mates a few times a season, but otherwise he is very, very average and if he were not one of Fergie's favourites, he would have been long gone.
Out of interest, which games has he outperformed his team mates in?