Because it doesn't always work like that in the real world where a folder might be shared with dozens of people. Users can, and usually do, leave a project in stages at least in fairly big organizations. But, yes, I suppose when the last one leaves the easiest thing to do would be to unshare the entire folder but all along the owner has just been removing users a few at a time. He or she would have to remember that when the last one goes you can't just remove him or her, and have the folder then appear as unshared, you have to unshare the folder at that point which is still just as silly as it was when I posted my first response a few hours ago, if not more.
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