Because of critical mass. A significant amount of non-technically inclined people use Windows. Some use Mac. And they're intimidated by anything different.
There's a bunch of non-web proprietary software medical offices use to access patient files, result histories, prescription dispensation etc. At least here in Ontario my doctor uses an actual windows application to accomplish all that.
Then they use those apps. The point is that since they usage of the OS as such is so minimal as to be irrelevant as long as it has a launcher and an X in the top corner.