For comparison, the Fate series did start with pornography and is easily one of the most popular franchises in and outside of Japan. I'm sure you've heard of Fate/Grand Order which is one of the most profitable mobile games out there.
For reference, a lot of popular anime series and manga started as pornography. Manga, anime, light/web novels, visual novels and games have a symbiotic relationship in Japan, so they get adapted from one to the others quite frequently.
As to why they do this, well, it's Japan. The average age is 48, and the average CEO is 60+. They don't really have the concept of fair use like in the US. IF you want a perfect example of how detached they are from the internet, you can just look at Nintendo who to this day still goes hard after Youtubers who just cover their games. They're extremely protective of their IPs, especially when it comes to foreigners or people in Japan whom they have no professional relationship with.