Isn’t mastodon basically centralized under mastodon.social? I’m not participating in microblogging part of fedi so I genuinely don’t know the situation there, but the statics on fedidb.org shows that mastodon.social has almost 4 times more users than the second largest server.
On Mastodon there are quite a bit of smaller instances that silence (basically opt-in federation per follow, one step before full defederation) .social/.online/universeodon and similar large instances. With the centralization we have wrt lemmy.world you cannot do something of that scale without significantly hurting your view of the threadiverse (and there are instances, most famously Beehaw, who have found that an acceptable compromise)
The community/group aspect of Lemmy also amplifies this centralization even more compared to Mastodon which does not have that functionality (without external bridges such as a.gup.pe) (yet).