Part of the reason are the different attitudes and work styles that exist towards schedules as stated in the text. Schedules may be viewed differently in other cultures as well and Project managers must be aware of these concerns. Sometimes the original schedule was not realistically thought out and was too aggressive. Time goals should be s.m.a.r.t. along with the rest of the project goals. Also, milestones can be used so that the project has points to assess the progress and help determine where the project is in relation to the schedule goals.
Another reason is that time continues to move forward no matter what is being done to manage it. Careful consideration must be exercised to keep this perspective in mind during the time management process. Project contingencies will arise in the development of complex systems and these will need to be addressed. Often though the Project manager has not planned for the unexpected events on some level and may misuse resources to address "Murphy" as Goldratt states. But project managers must be aware that unplanned events will happen at some point and there should be a plan in place to deal with the problems.
Also, people tend to build in too much safety time into their task completion estimates, but fail to utilize this allotted time and succumb to student syndrome, which basically means waiting until the last minute to begin the task and rushing through it quickly. This misuse of the available time causes projects to fall behind because too much false