There's plenty of reasons to not want others looking over your shoulder that have nothing to do with goofing off. Private emails, even totally business related ones are just that, private. If you're struggling with a problem, having an audience can make it that much worse. Et cetera, et cetera.
The nature of software development makes it hard for a lot of people to do with lots of distractions around them.
- There's also way too much stigma against "time wasting." Your brain needs breaks and distractions to work through problems. Spending some time on a goof off site each day is not a bad thing in and of itself. If your productivity suffers, that's a problem that needs to be addressed. But it can happen via burn out just as easily as via spending too much time reading fan theories on Infinity War Pt. 2.