And you would be correct for the most part but for development at least (and undoubtedly for other jobs but I'm a programmer so I'm going to focus on that) I don't just work between the hours of 9-5. I can't stop myself for thinking about work problems from bleeding over into my personal time. Likewise I cannot function as a human being in the United States without using some time that most people would consider "work hours" for personal matters. A number of gov entities are only open while I'm at work (Some even only make their website available from 9-5, a cardinal sin if there ever was one), things like banking need to happen within certain hours or they get pushed to the next day, etc.

Furthermore I use my personal computer as my work computer so there are apps/notifications that are not of a work-nature and are not for my coworkers eyes. Also I don't know if you have company chat or use some chat program at work but I'm guessing you would not like all of your chat's to be public or even just public with your co-workers. What I'm trying to say is I get where you are coming from but I don't think you are considering all the things you might do on your computer that you might not want to hide but at the same time you don't want to advertise.

More and more what happens on my screen is an extension of my brain and so you could say watching me use my computer is watching me think (from the searches I do, the docs I look up, etc). No one would consider giving a running monologue of their thoughts throughout the day but watching my screen get's pretty close to that, for me at least.... I may be an edge case but I think that I deserve some level of privacy. If you want to know what I'm doing ask the PM (or look on my whiteboard) or look at my commits or just ask me, you don't need to watch over my shoulder to know what I'm up to.