More like the part of Microsoft that implemented non-blocking I/O for Windows some time ago never bothered to tell the part of Microsoft that writes the generic Windows UI code for things like the open file dialog. Or for Microsoft Office applications, for that matter; I still see Word and Excel block the UI thread when opening a file from a network drive, even though Windows has perfectly good asynchronous file I/O API calls.
More like the part of Microsoft that implemented non-blocking I/O for Windows some time ago never bothered to tell the part of Microsoft that writes the generic Windows UI code for things like the open file dialog. Or for Microsoft Office applications, for that matter; I still see Word and Excel block the UI thread when opening a file from a network drive, even though Windows has perfectly good asynchronous file I/O API calls.