HTTP is a massive standard that takes thousands of man hours to implement. It's not something that "just exists" out there. I guess maybe the intent is that if you don't have Google writing your standard library you shouldn't even be able to check your mail.
I'm not a huge fan of IMAP (I wrote a client implementation for IMAPv4 a long time ago) but it's ridiculous that using what amounts to "SMTP for hypertext" as a transport layer is now common practice.