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.
Doesn't necessarily have to be the same. Afaik the signal protocol is for sending messages, not for transferring backups of chats.
Whatsapp actually lets you back up all your chats, unencrypted, to Google Drive or iCloud. Definitely not the same as Signal.