We thought of that a lot. But like our other baby, http://id4me.org - where we chose use use DNS and OpenID and mesh it into a federated identity protocol - email infrastructure is simply there, it's federated, robust, run by millions of servers, large and small, tested, filtered, pounded, drowned, abused you-name-it. That buys some extra overhead.
It's true that email is really the number 1 mean of communication, but I wonder if it's not ton and ton of added layers to make it modern enough.
And as it was mentioned, I'm not sure email is really good enough for mobile and other constrained bandwidth.
I might be wrong, but email headers are pretty scary.
In the end, I wonder if any old protocol shouldn't be upgraded, things made deprecated, etc. Every decade or so, protocol should be sanitized.