That's sort of what /all is. Unless you mean something like having a /lemmy community that discusses lemmy meta in general. Reddit actually had no communities / subreddits when it first started. That eventually turned into the /reddit subreddit as one of the many meta communties, which was then eventually set to read only, if memory serves.

I think this style platform has better engagement when a community has focus and direction, which is why you see smaller communities stay active and on topic here.

I don't think there's anything stopping anyone from undertaking the task of starting a general topic community, but on the same token, you need some direction and focus, otherwise it's probably going to be a challenging mess, both in terms of content and engagement. This is where reddit subreddits, and ergo, lemmy communities, originally came from. Prior to subreddits, it was just one big posting board like /all, which eventually led to issues like spillage and brigading and private communities.