Because the original posts are usually worse than the comments, so people talk about the comments. Most posts are just some crappy blog who wrote about a topic

Because the original posts are usually worse than the comments, so people talk about the comments. Most posts are just some crappy blog who wrote about a topic they just learned about or equivalent, while in the comments you see people who know what they are talking about and therefore don't feel the need to write a blog post about it but spending a few minutes writing a comment is fine.