Email is another decentralized, distributed system that is often ignored. The only real difference between chat and email is that the interface makes it seem like chat is quicker. Nothing stops us from writing such interfaces for email as well, though.
I think some distinction is needed: The email protocol allows decentralization (in the sense of federation) and, theoretically, even full distribution if everyone were running their own mail server. But in practice, none of this is the case and email is one of the most heavily centralized systems humankind has come up with.
Back in 2016 I came across a computer science paper where the author had studied the decentralization vs. centralization phenomenon in various cases (not just in IT but also in real-life social networks, economic dependency graphs etc.) and concluded that every man-made decentralized system will eventually exhibit centralization to some degree as it will develop nodes that have orders of magnitude more edges than the average node. It seems to be a very natural phenomenon. I wish I remembered the author or the title…
Yes, email and IM do have a lot in common -- in particular, XMPP is best visualized as an IMAP-like client/server protocol plus an SMTP-like server/server protocol that both share an XML schema (ugh) for describing the messages akin to how MIME (another ugh) functions for email.
As another commenter has pointed out, Delta.chat shows just how closely an email client can mimic the chat/IM interface.
One problem with email is that it lacks typing indicators, read receipt, and other niceties that people have become accustomed to. There can also be a bit of a delay between sending and receiving messages on different email providers.
