Because you're doing the same thing every day !!
I propose a new model for professional life, and this model can actually work in today's world where many people are multi-talented and some are even following this model, thereby making their office or work life less boring and more exciting.
And the idea is... to have more than one job! In fact, I take this a step further... no moonlighting please, but more than one job during the daytime.
Take my case: I've been trained to be a software engineer and I have some research experience, so I can play two different roles in a company. Moreover, I also write fairly well, so I could also participate in being a Technical Writer. I have been attending Toastmaster's meetings so in few months time I could also be considered to become an Evangelist. The idea here is that one can don many caps being in the same company.
Two important things to keep in mind:
1. Each employee must be given a very small chunk of the project (because instead of 10 hours they'd spend some 4 hours on that project), which means there has to be a person who must understand the project on the whole and can suitably divide the project to different employees so that their work can be easily integrated later. This person will only have the work of dividing the project and assigning it to others. He may then participate in other work, like some managerial aspects or be an Evangelist for a while.
2. The sense of urgency must go. In fact, less work hours for an employee on a particular project would mean more employees, so just before the deadline, all the employees could be asked to work few hours more to help the project launch successfully!
The payroll maintenance would become difficult in this situation but its worth it because an employee would gain different kind of experience, which means the company will have multi-talented employees who are ready to take up any role in the company, if the need be! This will make their life exciting and company's work easier, except the payroll part! Please note that an employee could have fixed salary or could have salary based on number of hours worked.
The latter case will also give the employee an option of working elsewhere for rest of the day. Commutation has to be worked out but if the two (or more) places are nearby, then the day can be divided into number of hours to be spent at each place.
Such a lifestyle can ensure less monotonicity in life which means a person will also be willing to work on weekends, and instead of lots of money from one job, will get some money from more than one job. Moreover, each person would be versatile, multi-talented and, lets be honest, productivity will enhance this way!
Besides, who needs a vacation when so much would be going on in life?