By growing large you attract predators and parasites, you must pay a very high price in moderation to removing this spam and trolling. An individual/small group of individuals fighting against every bad actor on the planet. By staying smaller you are more likely to fall under a 'dark forest'. You'll still be attacked by your local level trolls, but the likelihood you'll be attacked worldwide is lessened.
I also don't like the convergence of online identifies with real identifies in communities which also tends to ruin the point of an online community for me.
So having different groups for different purposes is precisely what I do want and having every human on Earth on one platform is the exact opposite of what I would want.