There are a variety of reasons, but one missing from the list is Word's utterly terrible formatting system that frequently breaks and never improves. Having used Word for years now in a variety of work applications, the inconsistency of formatting, the lack of portability Its strange what having a familiar interface, a grammar checker, and not much else can do to make a text-writing software a business necessity.
If ditching remains a wish and not a reality, some tips on coping.
Update: Found a grammar checker for OpenOffice. Yes!