The clients doing SMTP themselves allows them to communicate sending failures to the user. How would you do that for an email just sitting in an IMAP folder? You’d need to define and integrate some “sending” protocol into IMAP.

Furthermore, there are many use cases where clients only send, like notification emails or sending reports. Those use cases would become more complicated if those clients were forced to use IMAP. For one, you wouldn’t want to give all of them access to all email in other folders — or even to emails in the outbox sent by other clients.