Because you have a bias. I have a bias, too. There's nothing wrong with that - it's natural. Yet the folks at Politifact don't?"Mostly true" is one tick above "half true". Pretty close. I guess they could have rated that statement as "half true". But they did make the point that it was not entirely Obama's doing.
I think Romney's attempt to pin job losses on Obama that he had no control over is more deceptive. Rating it one notch lower seems reasonable to me.