I think it's a simple matter of focus. One could extend your logic and say 'Why don't you get your tech news from NY Times instead of Hacker News, since they have a tech section in their website ?'
True, but making a NY Times-like website for tech news wouldn't work either. HN works for techies because its clean and simple, and the karma/upvote mechanism keep the content relevant. So there is a difference from NY Times.
My question then remains, what is different?