I think this depends very much on how you use the tools.

My experience with email is that people have subject lines, email explicitly identifies to and cc recipients; email is threaded; email often has quotes/excerpting/highlighting from prior parts of the thread.

On the other hand, most chat usage I see is dependent on temporal aspects for threading (people under-utilize platform features for replies etc), tagging is generally only done to ping people to attract attention, chat groups are frequently reused for multiple different purposes.

Leaping to a point-in-time within a chat stream is often a bad user experience, with having to scroll up and down through unrelated stuff to find what you’re looking for.

Stuff in email is just massively more discoverable for me.

No, it has to do with context. In an email you will frequently have to provide more context for your answers to make sense. Chat is a conversation, which search drops you straight into, may with AI you could get placed at an appropriate starting point, but you're still reading a conversation. It's much easier to get dropped into a correspondence. To me the difference is like reading someones letter, vs. overhearing a conversation in a bus.

This obvious assumes that who ever wrote the email isn't a madman, who insist on using emails like it was a chat.

What makes email more useful in general is that each email is a separate object that you can organize in any way you want, i.e. move, copy, rename, sort into folders, attach as a file to any calendar entry, todo item, etc., or indeed to any other email. You can forward them to any other recipient, you can add and remove any recipient to and from the conversation at any time. It is conceptually powerful and flexible in a similar way that files in a file system are a powerful and flexible way to organize data. And it is easy to understand.

While all of these features could in principle be realized in a chat system as well, in practice they don’t provide that flexibility and power.

Another usability feature of emails is that they have a subject line. This allows to meaningfully list emails in a compact fashion. In a desktop interface, you can easily view and visually grep 50 emails or more at once in a mail folder or list of search results (in something like Outlook or Thunderbird or Mutt). This allows working with emails more efficiently than with a chat view where you can only see a few messages at once, and only of the same thread or channel.

Yet another usability feature of emails is that each email has its own read/unread status. This, again, is facilitated by each email being its own separate data object, and by the separation between subject and body, which allows the read status to be unambiguously bound to “opening” the email, or to navigating in the list of emails alongside a preview pane. And you can mark any email as unread again. In chats, the granularity of read/unread is the whole chat, whether you’ve actually read all of it or not. You can’t easily track what you’ve read or not in an automated way as with email, other than by that coarse-grained linear time-based property of when you last visited the channel.

Personally, when I send an email, I feel less time pressure to respond, so I more carefully craft my responses. The metadata is similar enough, but the actual data in email/forums is usually better.