iMessage chats also include rich media that is either degraded in MMS (photos, videos unless you have RCS support) or just doesn't exist (like multiplayer games, invites, apple cash, etc).

This may not seem like a big deal to you, but if you remember what it's like to be a kid, you should get it. The smallest friction can be a reason to exclude someone socially.

Imessage already compresses it to hell enough. You need another protocol for fidelity.