They're not just staying on those instances tho...

When they get defederated (usually long before) they find a small boutique instance (that's actually valid) but didn't bother to set up any requirements for account creation then make a shit ton of accounts to troll with.

If you ever look at the main modlog you'll see there's a significant amount getting banned on a daily basis.

But they just go make another, and even tho most instances are doing something about it, there's a surprising amount of zombie instances no one will ever change but are still running for some reason and federated with large instances.

Eventually the main ones will need to "prune" a lot of instances, which shouldn't be surprising considering a year ago everyone wanted to federate with everyone.

Imagine just walking thru a city and inviting literally everyone to come hang out at your apartment, even giving them the door code to get I to your building. That's kind of what happened.