The answer, of course, is very simple: it's because you, who know the correct spellings, grammar, etc., have idly passed by mistakes when you saw them. That's right, you could have corrected them simply by clicking on the "Edit text of this page" link and making a quick change, but noooo. You'd rather just complain, instead of doing something about it!
You disgust me. Now go out there and do that copyediting!
Uhmmm, actually, I copyedit these pages all the time...
Just changed 'who know' to 'who knows'. Done my share.
Just changed 'who knows' back to 'who know', so that the subject ('who' representing 'you', therefore second person) can have a verb that agrees with it. Did my share.