Because sometimes there are many tasks that need to be done, and maybe they could not guess what is going to happen is what the obstacles that make the job done according to a specific period of time.
Sometimes there is more work to be done. And sometimes unpredictable obstacles that would have happened on the job not completed by the specified period.
Because sometimes there's plenty of work to be done, and sometimes may not predict that there will be obstacles, what happened to the job was done according to the time limit.