Actually just took a college test on this. So there are two reasons really. It's so far in the future you don't see yourself getting the reward anytime soon. Therefore you don't care cause it doesn't affect you positively in anyway in the now.
Or Second, because of scalloped response patterns. Like I get paid once a month. I work really hard close to the paycheck date because it matters to me more. Then right after my desired behavior plummets.