I'm not sure if you are being serious about not understanding "the whole centering a div meme". Your example handles a trivial case, but does not address the whole of the problem.

As others have pointed out, vertical centering is often the problem being discussed (although difficulties with horizontal centering do happen). Anyone I know that has written any non-trivial web application has run into the situation where they spent way more time than they thought they should have to getting some element in a web application centered on the page the way they wanted it to be.

This article is a good example of the complexity, I think:

https://css-tricks.com/centering-css-complete-guide/

The author makes a decision tree, which illustrates the complexity fairly well, and then there's a conversation in the comments between the author and a reader about whether parts of the decision tree are correct.

CSS is extremely complicated. It's easy to get lost in the complexity, and it can be very frustrating when you know how you want something to look, but can't quite figure out how to get it to happen.

That's why the meme is so popular. LOTS of people who deal with CSS can relate.