There may be a performance/efficiency motivation for this.
Lemmy communities work by having a psuedo-user that “boosts” all posts and comments it receives to all it’s subscribers, meaning all instances are aware of all comments (as long as at least one user is subscribed, and barring any defederation). I’m not entirely sure on what the “reply fetching algorithm” of Masto is, but it doesn’t go out of it’s way to fill everything.