On February 15th, newly-created Fediverse accounts started posting spam messages from various instances, sending invites to a Discord server for a Japanese troll organization. This spam was widespread across the Fediverse. The posts frame ap12 from “KuronekoServer” as the culprit behind the operation.
Looking at their spam content (in Japanese)
It’s probably more expensive and inconvenient.
Also it might only take one report for an active mod team to ban a server. How long can that take? An hour? Less? If they’re on servers that real people use, bots have to be banned one by one, so the spam can last a lot longer and reach more people.