Because you are distracted!
Even right now as you're reading this, your mind is wandering. And you are not sure why you are reading this in the first place.
Perhaps you are supposed to do something else. Something that you need to do in order to be productive, but you just don't do it.
You lose your focus because you are distracted. Distracted by the small things, the unnecessary, the unimportant and the unknown.
In order to be productive, you need to get your focus back.