They could, but I don't think it would really add much benefit. One of the main features of Slack vs IRC is that Slack has persistent and consistent chat history. If you look at apps like Signal, when you log on from a new device your history isn't available, because of the end to end encryption used to store the messages. To view the history you would have to be able to decrypt it, but that means there has to be some mechanism for users joining a channel for the first time to decrypt the history of that channel (unless you want to break that functionality). Unless there is some really neat cryptography trick I'm not aware of, that would mean you can't have end to end encryption where each user's data is encrypted in a way that is opaque to everyone except user(s) in the chat like Signal or Telegram.

Now I guess you could set up some kind of broker system and have like a different encryption key per channel or per conversation or something, which would at least mean some hacker has to get both the archived chats as well as the keys. This would make stealing chat histories more complicated, but not less possible.