To be fair, the goal of the tutorial is not to lead you through the steps to produce Outlook in the minimum number of pages, but rather to provide a comprehensive learning guide to our designer and developer tools while building a fun
and useful sample.
(And that's not to say that building an Outlook-like interface is a trivial task, either - particularly if the controls are real, working, data-bound controls. I'm sure it was significantly more work for the Outlook developers to build this code in Win32.)
I suppose the logic here is that "in the era of 1,000+ page programming books, what's another 90 pages?"
Out of curiousity, how hard is it to write your own WPF controls anyway?