Chat is inherently in flow because you can’t manage the read/unread status per message, and you can’t move messages to different folders. When I check for email, I might have a dozen new messages, clearly listed one per line, and I can pick which to read now and which to read later. I have a clean overview with the mailbox listing and the read/unread status. I can easily overview 50 or so messages without having to to scroll. I can archive the messages I’m done with, while keeping those around I still want to handle.

In a chat, the read/unread status is not per-message. It’s much harder to discern separate exchanges within the same channel, and to handle them out-of-order when some are more urgent or relevant than others. They also take up substantially more visual space than a mailbox listing, so you have a much smaller “peephole”, making it more difficult to get an overview of what is going on in a channel. All this has the effect that people treat chat channels as a single continuous flow of messages that you catch up with in the order they come in; and the messages that scroll out of view, which happens fast, tend to go out of mind as well.