Because, you know, nowadays nobody has a smartphone or any other devices capable of browsing the web besides their PC...
I'm starting to think that MS secretly wants desktop Windows to die so they can concentrate on server-land OS roles (especially Azure based), Office, and SQL Server. Once their dev tools and office are cross-platform and/or web-based (they seem to be moving that way) they can push people onto Linux/Mac/Other on the desktop and let someone else worry about the intractable problem of maintaining the security of the desktop/tablet OS installs of the filthy masses...