It&#x27;s because of the UX. Email has traditionally been letter sized messages, each message takes up your entire screen or some other large fraction. Email for most people is HTML flyers sent to them by companies so it would be difficult to render a message and letter based UX with the same standard.  The amount of friction to send a message is pretty high too.<p>Also because of the design of email, the speed of interaction is pretty slow compared to a message style UX. The creators of these apps favor low bandwidth network communications, so sending the word &#x27;ok&#x27; to someone doesn&#x27;t involve a large header, the rest of the conversation quoted in the same email.  It&#x27;s just the word &#x27;ok&#x27; and maybe a conversation&#x2F;user id.