It's upvoted because, after people post a comment utterly destroying it, they want their comment's unique wisdom and intelligence to be visible to others. Even (maybe, especially) when the original article is garbage.
Well, i will add that the major determinant for whether an article stays on the front page is whether there is an active discussion.
So the more chatter there is, the longer it stays on the homepage. The longer it stays on the home page, the more people upboat it without reading the post or the comments.
But that's me retreating to the study of aggregate behavior. sigh
It's a perverse incentive problem in social news.