This is true, but of course individuals can choose to move.
This may not be a warm fuzzy thing to say, but I don't feel much of a "community" in reddit subs.
What I mean is that to me, 100 people reading a sub on lemmy once a week is just as useful as 1000 people reading a sub on reddit once a month. As in, I don't really care if the specific users from a reddit sub are here, just that there are some engaged users here.