There are plenty of threads that aren't news or events that are posted on this subreddit and create discussion. Besides video essays and articles there are text posts where a user talks about a subject and gives their opinion and opens the comments for further talk. I recent post that sticks in memory was about "not knowing how many pages are left in the story" using the Witcher 3 as an example.
But yes these posts are rare, and many of the ones that are created get deleted. There is a reason for that.
Often they are deleted because of rule 3. A text post made up of a single question will get automatically deleted.
As you wrote; most other posts are deleted because they violate some aspect of rule 7. 7.1 and 7.2 prohibit questions that are too specific or too broad respectively. A small while ago a post asked "What's your favorite video game weapon?" Which was too broad and was deleted. It was obvious that the question was too broad because the majority of the comments were just one word answers. That isn't discussion it's just an open poll. People have argued that it's too difficult to find a balance between rules 7.1 and 7.2 and that they shouldn't be rules. I would say that while yes they are difficult it still isn't impossible and the quality of the posts that survive are proof that the harsh rules create good content.
And that's the most important thing here. This subreddit is about quality not quantity. There aren't that many self posts but the ones that stay are great and have lots of good discussion in the comments section.
You write "Moderation is supposed to be about getting rid of the content that doesn't belong here." But that is a gross over-simplification. Moderation is about keeping the subreddit true to itself. There is a reason that this subreddit isn't a default and doesn't appear on /r/all. There is a reason why votes aren't what determines what stays and what goes like they do in /r/gaming. There is a reason this subreddit is strictly moderated.
You don't have to like it. This subreddit is a community not a prison. There are other subreddits that have what you want and you can frequent them and still come back here to catch up on gaming news and comment on one of those rare good discussions that abide by all the rules. And maybe you can appreciate that those posts exist at all even if they are rare because I guarantee that you'll never see anything similar on /r/gaming.