Of course they could fix MS Word's HTML output and fix Outlook to use MSIE's rendering engine ... but that'd only reduce lockin by allowing Outlook users emails to be read properly in other MUA. Never going to happen.
I believe up to 2003 this was an option that was not enabled by default too.
e.g. an known Outlook 2010 bug (with Exchange 2013) crash the Outlook client on startup. But you can still open *.msg files, it opens the mail in a window and one can edit the mail.
It's all about Bill Gates vision "information at your fingertips" from the early nineties and the Cairo operating system.
Watch this video from Bill Gates Comdex '90: http://youtu.be/kL8zoQVJaD8?t=19m26s
Cairo '91-'96: https://en.wikipedia.org/wiki/Cairo_operating_system