Yeah in SEO it's widely known forums are easy to outrank

It's because the content is usually shorter and less in-depth than a blog post. It will also contain unhelpful posts mixed in with helpful posts so the user experience is not always great

Imagine a question followed by 1kb of HTML containing a username, avatar, signature etc just to contain the words "I'm also having this problem" or a meme. Sure 20kb of HTML later maybe somebody gave a helpful reply but it's just noise. Now compare that to a properly crafted blog post. What do you think Google will choose?