This MIGHT be because the invite/accept/scheduling setup in Exchange is so good that it's hard to sell anything else. It also might be that most big orgs are very wisely reticent to host their email with an advertising company.
I'm also gonna push back on the "web tools are enough for most people." I mean, how effective is the web version of Excel, really? Can it easily interact with local data stores for elaborate data mining and updatable queries? My guess is "probably not". How is it for macro development? My company works closely with the finance orgs of our client parties, and let me tell you a LOT of these folks are extremely advanced Excel users. Web tools aren't to par here.
Web Outlook is also pretty awful compared to the native version, quite honestly, as is Word. Maybe PowerPoint is okay, but I'm not a power user of PPT so I don't really know.