Simple. MS doesn't believe in those superstitions, but there are people who do. Subsequently, those people would not buy Office 13 or Windows 13, which is lost revenue for MS. Plus you know ever blogger out there would go "see, bug X is evidence that Office
13 is cursed!" Much easier to avoid any trouble by just numbering it 14.
And the number 7 is a magically powerful number and is good luck too. In China, the number 4 is bad luck, because in Chinese, it is similar to the word for death. I bet that factored somewhere in the release of Windows 95, which is technically Version 4
of the Windows that used DOS as its kernel.
With 7 being magically powerful and good luck, maybe Windows 7 will do really well?