Because it actually is a fair-and-balanced show, unlike, say, ABC/NBC/CBS/CNN or Hannity/Rush. If you only want your viewpoint parroted back to you watch one of the other shows. O’Reilly is the show to watch if you like hearing debate and a variety of opinions on an issue. The typical trope for O’Reilly-haters here on FR is to post an opinion of O’Reilly’s that they disagreed with and then claim he’s an idiot and they’ll never watch the show. Which means you simply miss the point of a fair-and-balanced show where you’re going to be exposed to a mix of opinions you like and ones you don’t. With O’Reilly, you know you’ll almost always get to hear both opinions on an issue in a debate format. Which is what makes it one of the most interesting and entertaining news programs on TV.
Actually all you really hear is Big mouth O’s opinion since he overtalks all his guests.