Good question. I think it has multiple reasons.

1. It's not that all my posts are meta, but the meta one's are the most successful.
2. I want to contribute content to lemmy, so that it will prevail and attract new users. I also make memes on things that are on my mind, which is lemmy, when I'm on lemmy.
3. Well I could post memes about very niche topics, that I'm interested in, but I think that they wouldn't be interesting/funny for most of the users here. While content about lemmy is most likely interesting/funny for the users here.
4. I think shitposting is rather often meta.
5. In the last days not many things happened, that would be memeable.
6. So that can see that it's OC.

But I take your comment as a challenge to shitpost more on other topics.