When Nintendo gets litigious, it’s mostly out of fear. Depending on the way international copyright laws interact, you often can lose your intellectual property if you can’t demonstrate that you’ve consistently tried to defend it (that’s why Bethesda sued Mojang over the word “scrolls,” not because there was any real issue, but because if they didn’t, someone else might call a game “Elder Scrolls 10” and then point to the fact that Scrolls was allowed to go un-litigated and win the case).
Nintendo, moreso than any other games company, makes most of its money off of brand recognition-- they’ve been repackaging the same Mario game as new over and over for thirty years now, and it sells because it’s Mario. So I’m not surprised that they get very protective of what’s by far their most sensationally popular IP.
It’d be cool to have a 'mon-like game where your monstergirls(?) got fatter instead of evolving, though. I only have RPG Maker XV, which I don’t think has the Pokemon Essentials pack, and my code-fu isn’t strong enough to rescript RPGmaker combat (which is a shame because the default first-person-mode is the absolute worst).