Academics constantly complain about being overworked, yet they also praise their freedom to decide how and when to work as one of the great advantages of their job. This seems contradictory. If they have such freedom, why are they so overworked?
Part of the answer is of course that you are never really done. You can always do better. Write more and better papers, get better research results and deliver better teaching. But this is true for most white-collar jobs these days - you could always work more on your next budget, product description, market report, technical drawing or whatever it is you do. If you are good at what you do, it should be enough to work normal hours, go home and get some good rest and return refreshed next day to do more work. Few successful academics do this. There seems to be some extra factor that affect academics, contributing to that they always seem stressed out - and that you very rarely see successful academics below the age of 50 working reasonable hours.
That extra factor is increasing marginal work utility. Simply put, the more you work, the more you get out of each extra work hour. This gives you a very strong incentive to work as much as possible, which is clearly not a very good idea if you also want to live your life. Let me explain.
We begin by imagining I work the statutory work week of around 40 hours. If I work efficiently, I will then have time to prepare and conduct my teaching, supervise my masters students, do my administrative duties (including answering lots of mails), review papers for some conferences and devote the minimum acceptable time to supervising my PhD students. These are all tasks that I must do - not doing them would mean that I am simply not doing my job. This amount of work is clearly not enough, as I do not get time to do any research - though I could of course try to take credit for the work of PhD students that I don't supervise enough.
If I put in some more time, I have time to do some of the research where I am not in a "driving" role. I could participate some more in what my PhD students are doing, help my masters students write up papers based on their theses (which I have supervised), do my part in joint research projects with people in other institutions, and serve on organising committees of conferences. Also, write parts of collaborative grant proposals. These are activities where I am mostly fulfilling obligations to other people, though they are also activities where I get relatively much visibility from limited effort. But it does not mean doing any research on my own.
So let's say I put in even more time. I can now advertise some of the research I (and my collaborators) have been doing and help shape the directions of my own and others' research, for example by going to give invited talks, or write parts of survey articles. I can start new initiatives, e.g. competitions, discussion groups or grant proposals. These are things that make a lot of sense to do and I enjoy doing them, but it is still not the same as actually doing my own research.
Finally, when I'm done with all of the above and there are no urgent mails awaiting reply in my inbox, I could sit down with my own idea, write some code of my own, run my own experiments and write a paper with myself as first author. In other words, do some research of my own. This rarely happens, as I rarely get to this point and when I do I rarely have any energy left.
The utility of that sixtieth work hour is so much higher than the tenth or twentieth because I can use it do my own research. If I work 60 hours rather than 40, I don't get 50% more of my own research done, but almost infinitely much more, as I would otherwise get almost none of my own research done. Given that I am interested in doing research of my own, there is a very strong incentive to work very long hours. It is not that I am uninterested in any of the other parts of my job - I enjoy all of them, except grant writing and meetings with management - but I am more interested in research.
You could compare the job of an academic to having a teaching and administration job and having research as a hobby. Except that the "hobby" is the presumed core of the job as advertised, and the reason you applied for the job.
An interesting detail is that it wasn't always like this. Back when I was a PhD student, and to a large extent still when I was a postdoc, my marginal work utility was basically flat as I spent most of my time on my own research. But as I was good at research I got promoted to a position where I had to spend most of my time doing something else. (Incidentally, I have seen quite a few excellent researchers who are less than excellent at teaching and administration.)
Finally, let me point out that I am not complaining, just explaining. And I am certainly not blaming anyone, especially not my wonderful colleagues and students. After all, I love my job and I would not trade it for any other job. I just hope to have made the logic clear behind why I, and probably many others like me, work such long hours.