Sometimes less really is more

In Pilates, why do we generally do fewer reps than other forms of exercise such as gym work or aerobics etc.? Joseph Pilates would call lots of repetitions "mindless repetition ", he would say "always keep your mind wholly concentrated on the purpose of the exercises as you perform them."

It is the mind that moves the body! The demand is on your mind while you move through the exercises, not your body. Your body can keep on repeating and repeating a movement, while you focus on your shopping list or the meeting at work that is coming up next!
But you're only moving in your habitual way. So, if you want to improve your posture, or realign those tight shoulders that give you tension headaches or soreness after you've been on that computer, then focus your mind. Become aware of your habits and concentrate on the movement.
The mind is where the most profound changes occur!  Less is best. Awareness is key!