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Short answer: MS Windows is not Posix conform.
Flame answer: MS Windows steenks.
Long answer: Maintaining code for a platform that none of the developers use is a little hairy. Besides, even if that were case, writing POSIX-conform code while simultaneously regarding the madnesses (yes, there are many of them) from Redmond, and preferrably sustaining the perfomance is absolutely impossible. There is much code which has not been optimised (or sometimes even de-optimised) merely to take some windows concept into account.