Answer:
Because we are humans and not machines.
Step-by-step explanation:
We are all different, each person has his own way to study and learn. Is very important to take some breaks, some people can pay attention for many hours and others just for a few minutes. You need to know yourself and calculate how much time you can pay attention to what you are studying and how much time is enought to take a break, that's a good way for your own organization. Procrastination is not the same than a break so be careful, sometimes procrastination tires you more than sitting and studying.
Good luck and hope this helps!