Because if they got their job done and left they would be fired. If they got their job done, they would be given more work. So one takes the maximal amount of time to do a task given the requirement that they be in the office for 8+ hrs a day.
If they have 8 hours of work time and finished their tasks in 6, does that mean they have exhausted their usefulness to the business? Why not spend these 2 hours doing some small miscellaneous tasks?
Because that's scope creep and it can create the expectation that it's part of my job, so then I'll be responsible for it. Down the line, if one of those misc tasks becomes the ire of someone else and my name is attached to it (comment, commit, etc.), they'll contact me further down the line, perhaps when I'm dealing with something of my own. Then, at least, I have to explain that I don't maintain that and in the worst case (which has happened), I have to fight to not be responsible for it right then.
There's a balance in having your name associated with 'things' in a business. Some people might be good at being a know-it-all, being well-known, and handling everything (why aren't they running their own company?), but you'll increase your chances of interruption the more wide-spread your name is.