This is a good question. I sometimes comment about and help others on the topic of travel on Reddit, often writing a lot of content. The incentive for me is that it's fun and intrinsically rewarding to help others do something that I find meaningful (which would fit into sibling commenter Gatsky's reason #2). So in a way it's kinda self-serving, but in a good way.
As for why authors don't just link to another webpage: I think it's because a) those pages often don't fit what is being asked, if it's more in-depth; b) the author might have a way to explain it that they want to use; or c) you might presume that writing your own content will get more upvotes.
Meta-funnily enough, in this thread we have both approaches: myself and others offering opinions, another guy linking to a Quora question.
As for why authors don't just link to another webpage: I think it's because a) those pages often don't fit what is being asked, if it's more in-depth; b) the author might have a way to explain it that they want to use; or c) you might presume that writing your own content will get more upvotes.
Meta-funnily enough, in this thread we have both approaches: myself and others offering opinions, another guy linking to a Quora question.