From several people saying they had actions taken against them for reproducing those on Kickstarters and other places. It's absolutely true that you can't copyright mechanics - but WotC has frequently went after people for reproducing their CC rules in text - I think the legal argument they fall behind is that the CC of D&D is a unique enough process to fall under their IP, but I'm not really clear on the specifics - I just see people saying its a thing Wizards have went after people for. See this comment thread for more;
https://theangrygm.com/save-vs-lawsuit-how-to-publish-dd-content/#comment-29263
While you may be technically correct, it's probably better for most creators to ere on the side of caution in these situations - no one wants their big project squashed by a C&D they can't afford to fight.