Some information available on websites is outdated or incorrect. Or maybe many hubbers feel that they can say it better. There are many articles on hubpages - travel articles, tech articles, etc etc etc... that have been explained several times throughout the Internet, but people on HubPages continue to rank and make money. Maybe many hubbers do not have quality articles about a specific topic, like you said, but there are hubbers who do well with their hubs regardless of how many times a topic has been done. I have several SEO hubs up and all of them do well. As people continue to do well with a particular topic, there will be more and more articles popping up on the same topic.