From the perspective of market fit and success, there are countless less-technical business users who swear by Outlook. They are not familiar with anything other than the kind of endless thread top-posting in odd encodings that Outlook all but ensures users will do -- that's e-mail to them. Why stop selling it to them?
To be clear, I loathe all of it and I haven't used client Windows for more than a decade. When I used to support small business IT customers back then, Outlook was by far my least favourite tyre on the raging intergalactic rubbish fire. But it's not hard to see why they don't care that Outlook is the way it is.