There’s been a long history of them, and an entire industry doing things like filtering attachments or rendering HTML emails in sandboxes.
I think the original poster made an attribution error: iMessage gets attacked because it’s popular. If it didn’t allow you to receive rich messages from anyone, people would switch to other apps which do and there’s a long history of those being exploitable, too. What makes iMessage special is that you can assume an iPhone user has it enabled without having to check whether they use WhatsApp, Telegram, Facebook Messenger, etc.
I think the original poster made an attribution error: iMessage gets attacked because it’s popular. If it didn’t allow you to receive rich messages from anyone, people would switch to other apps which do and there’s a long history of those being exploitable, too. What makes iMessage special is that you can assume an iPhone user has it enabled without having to check whether they use WhatsApp, Telegram, Facebook Messenger, etc.