Many niche communities have more activity than lemmy itself or something rivaling lemmy. You're missing my point about lemmy not having enough people. Lemmy is literally carried by few select posters. What's worse is the fact that many communities straight up can't be carried by 1 person because it's for example a community for people's personal stories and just copy/pasting it to lemmy would kill all nuance to it.
It has lots of very active niche communities that lemmy just doesn't have. If you really want them then you have pretty much no choice. It's also still a very useful site for your browser searches because of the amount of information on it.
I've been avoiding it for years and finally gave in in the moment of weakness. I uninstalled it very soon after because I hated the experience so much and never gave it another thought nor chance. xd