Because it is wildly disrespectful to the original artist, coming close to you tacitly trying to claim credit for creating it, and makes it harder for people to find the original source, especially if someone quotes, crossposts, forwards or otherwise repeats your post.
It’s thoroughly obnoxious behaviour, you’ve been told that repeatedly, by many people. Just stop doing it.
I’m sure that’s a reference to something, but I’m at a Loss to figure out what.