Why aren't there more custom admin interfaces for WordPress?

I'm trying to think why I haven't seen more custom admin plugins and/or builds for the WordPress admin, especially now that the API is maturing into a full-fledged and stable interface?

Obviously, Calypso is awesome — but what about a simple interface makeover?

I'm mostly asking this to decide if I should start a side-project for this, or if there are good reasons it's hasn't been done yet.