While you may be correct, that was my experience. As a new user, I joined two Lemmy instances, was unsatisfied with the full feed on both, and said "screw it, I'm going to the biggest server".
The problem with telling people they can fetch the missing comms are threefold:
It becomes a perpetual maintenance task. New communities are being created all the time and I don't want to have to reference other servers' feeds regularly to stay up to date on the newest stuff. I might as well just be on that other server
Part of the joy of the firehose is seeing when some completely obscure community has a wildly popular post that one time because it's extra funny or shocking or whatever. Those posts just won't make it to most smaller servers.
It's an "unknown unknowns" problem. Sometimes you know what it is that you don't know and can go find it. But often I don't know which things I don't know, so I can't seek it out to add to my server. The beauty of a big server is that I don't have to do that legwork or even think about it.
All it takes is one user on the server subscribing to the Western Spotted Bull Frogs community for me to see it when they have a post blow up. The chances of one such user being on my server go way up here on lemmy.world. I'm sure there are smaller servers that are "good enough" in that regard. But why would I bother when I have what I want right here?
Not trying to be argumentative, just calling out what I see as a fundamental truth about Lemmy, compared to other fediverse applications. Like, on mastodon a big server's fedirated feed is more or less unreadable. That makes smaller servers appealing as it helps prioritize what makes it into the feed. On Lemmy, the voting system does that prioritization, removing one of the big reasons to avoid larger servers in the first place :)