Maybe because that is what the teacher is teaching and the student is not advanced to the point of knowing how to do free technique and reversals? Sometimes teachers don't want their students just doing whatever they feel is right because they want the student to work on something specific? Maybe the student does not yet have enough experience to be able to discern ,"what the situation calls for"?
All that and then some. Training isn't a self-defense situation, and must not be treated as such. When you're in class, you need to try to the best of your ability to do what is being shown, as uke and nage, and as sempai and kohai. Newer students should not try non-aikido techniques, or even aikido techniques that aren't the ones being shown; senior students should respond to the junior students in ways that will help them learn what they should be learning right now (i.e., how to do this technique this way). Sometimes that means pointing out an error that the junior is making, but only when the error is getting in the way of what they're trying to do right then, or creates a dangerous situation. Otherwise, zip your lip and let Sensei do the teaching.