The best part of the fediverse is that anyone can run their own server. The downside of this is that anyone can easily create hordes of fake accounts, as I will now demonstrate.
Fighting fake accounts is hard and most implementations do not currently have an effective way of filtering out fake accounts. I’m sure that the developers will step in if this becomes a bigger problem. Until then, remember that votes are just a number.
Then they will just distribute their bots equally to other legit servers, and by that, defederation is not a viable solution anymore.
One other problem are real human troll farms
If they can do that, they could’ve done it on a traditional site anyway
“Legit” instances are able to moderate/control the spam coming from their users.