So, im having a hard time finding the sources but my understanding of part of why they are so agressive is due to the way the law is applied in Japan. Apparently if they allow any type of content or creation and that content is granted either copyright or trademark and that rite exists in parallel to theirs it weakens their argument or ownership.

So if I make game play videos, claim they are mine, and seek copyright in Japan then Nintendo could lose their ownership as my copyright could be used as case law or evidence that Nintendo does not actually own say Mario.

https://youtu.be/i13hrynnGNY has some good details that help explain their “mindset”