If the question is "why not", I think a good reasoning is simply because it broadens the categorization to the point where the term begins to lose all meaning. Different websites inherently have different forms of structure for interaction.
The way you behave on Instagram, Facebook, tumblr, and Twitter all feel fundamentally different from the way you behave on forums and video sites. People on Lemmy don't "reblog" stuff, people don't send each other friend requests on Youtube. It redefines the core dynamic.
On Youtube, the dominating theme is channels and parasocial relationships, more similar (but not exactly the same) to an audience's relationship with traditional media like Film and Television and their responses to the various actors/artists/celebrities therein rather than a 1:1 reciprocal relationship. Sure, there are fan groups, which gets into an interesting subset where you can begin to socially interact with each other, but these groups often live in outside spaces (such as tumblr). Hence the point I'm trying to make.
Even classifying Lemmy and Reddit as "social medias" feels like a stretch. I mean yes, in a technical sense we're all "socializing" with each other in this moment and having (mostly) genuine conversations. But for the vast majority of it, I would say:
Tiktok is the definition of social media imo. You respond directly to each other with videos that reveal who you are in real life. It's almost aggressively social lol, like neighbors yelling at each other inside of the same apartment building, even if you live on opposite ends of the continent.