While browsing recent deletions, like one does, I found a recurring deletion reason being "Artist's own artwork". I didn't understand what this meant and was intrigued. I looked into the guide on what to take into account when uploading and read through the terms of services, but still couldn't quite figure out what it's supposed to mean.
Is this a copyright infringement issue? Are people not allowed to post images they created themselves? If the latter is the case why is it forbidden and why was I unable to find this rule in the upload guide or the TOS?

It's the latter. And I guess it's mostly to prevent self-promotion from subpar amateur artists hoping to gain an audience. Which is what most self-uploads tend to be, from my expeirence.

That's understandable.
I'm still not able to find where it says that in the rules though.

https://yande.re/forum/show/18334 Checkmate is an admin, so it's basically a rule.

Yandere like Danbooru is a curated image board and Danbooru explains it this way:

Posting one's own art is severely frowned upon, and there are two reasons behind this:

Firstly, artists tend to be biased when it comes to judging the quality of their own work. What looks good to you may be regarded as mediocre to Danbooru moderators.
Secondly, allowing posting of own work (self-posting) will cause a sudden spike of sub-par uploads as users mistake us for another Pixiv or Deviantart, far beyond the scope and capacity of Danbooru.

More suitable sites to upload your own art to include Deviantart and Pixiv.