It's appification. People want an app. And easy. Things like imap, smtp, pop, certificates etc are hard, too hard for most.
And email, done right (safe, no spam, secure, private) is one of the hardest things to do. It's not easy.
And because it's not easy you see everybody moving to cloud for email.
Do you have a point? Oh yes. But can you, do you want to explain setting up thunderbird to the median college goer? Can you do that 50 times a day? (yes, click imap, no change the port to the secure one, what a port is? Dont worry, just change the number, oh you typed imapp.Server.edu, no that should be imap, one p. No, not b: p. Yes. No. Sigh, just come over I'll type it in. FFS.
That's why the cloud email services are everywhere. Email and users just dont mix well.