When a spammer posts many times in different communities on multiple instances it’s much much less work for everyone if the admin on their home instance nukes their account and all their posts and have that automatically propogate to all instances.

The alternative would involve admins/mods on every instance that received spam having to remove their copies. On many smaller instances that just wouldn’t happen and their copies of the spam would be left online forever. That would make the fedi extremely attractive to spammers and even more would turn up. We’d effectively have turned the fedi into a spam amplification botnet.