In every sim league there are a few teams who are rebuilding. Not rebuilding like a ML team would, managing to keep enough decent players on hand to win 60-70 games, but totally jettisoning any good players and not caring whether they lose 110-120 games(or, some would say, trying to lose as much as possible). They're in every league.
My point is that hitting is much more consistent than pitching, so that a team looking to rebuild will most often trade pitching, because there a very few pitchers who are good consistently year in and year out. Pitchers are also more likely to get hurt. So they dump any and all one-year or short term pitching, which is most middle relievers and many starters. But they hold onto their hitters, except older ones or people like Galaragga who won't be there next year.
So the good teams have good hitting and good pitching, but the bad teams will almost always have good hitting and terrible pitching. It would be intersting to contrast the bottom 5 teams ERA in MLB with the average PB league. I think the PB leagues would be much higher, not because of game prejudice, but because of the awful(often innocuous) pitchers who rack up 180-200 IP in PB and half that in MLB.

Just a thought.