Different contexts have different threat models. If my goal is to have a secure, private conversation with someone, I'll use Signal. If my goal is to communicate some less-than-sensitive information with someone, but the content isn't relevant to anybody else, then an unencrypted DM is fine.

In the context of public-broadcast social media, the service's ability to moderate abusive uses of a DM system is probably more important to me than the ability to have absolute control over who reads my messages.