Because there is also professional benefits about writing with your real name. For example the famous patio11: https://www.kalzumeus.com/greatest-hits/
He also goes into detail in this podcast about the benefits of writing with your real name too: https://player.fm/series/north-star-podcast/patrick-mckenzie...
Funnily enough, he also lives in japan!
If you post something in your real name, your colleagues WILL find it out and your boss WILL read it and your neighbors WILL scrutinize it no matter how woke your post may read. There WILL be consequences from being pointed at to losing your job and pension.
People using real names for app reviews, online comments, those are completely insane to me. Including shorthands like “John D.” Unless you are paid to do so that is.