Very much this.

But also, just having a huge number of people on one platform makes for a lot more content on obscure topics. You can have a niche interest and find a whole-ass subreddit full of people with the same interest. There are definitely subreddits I miss. But not enough to wade through the cesspool that is Reddit to pick out the few remaining gems.

Yes, we can make similar communities on Lemmy, but without the sheer volume of eyeballs that Reddit has, there's next to no chance that such communities would take off. (I'd love to see an equivalent of /r/bestoflegal, but the only way that's feasible is for it to copy content from Reddit, which isn't really the solution I'm hoping for. /r/fuckhoa is endlessly entertaining, but again, way too niche to take off on Reddit. I think you all get what I'm saying here.)