Why not? It's likely because the platforms were all much simpler then. Vendor-specific features like stickers, end-to-end encryption, video attachments, reactions make it difficult to support a subset which is going to satisfy regular users.
RFC XXX Reaction
RFC XXX media inline as attachment
Of course we know why, and it is all about who owns the path to people's attention.
I am waiting for SPAM levels to come up and little breakages to begin. Email is a lot like the Post Office.
Everyone imagines it is them in front.
Sigh.