I think the point is no one can predict who will have a good or bad year. However, in franchise mode, you have a lot of options to affect that. You can promote or demote players if they are not over or under performing at their level. You can set training options to address attributes that are dropping. You can change positions, which can increase or decrease their rating. You can look at morale to see if they are wanting more money or playing time or whatever which will change their rating. Sometimes players just want to be close to home, or have enough players from their country on the team, so a change of scenery can greatly affect their performance.
I get not everyone wants to deal with all that and being able to shut it all off would help some people and not hurt others. But seeing as how there are so many mechanics built around player progression, I can see how they don't want to disable the majority of the mode's mechanics.