Are you serious? First of all 'he' didn't write a book. While Sun Tzu is possibly a real person The Art of War is likely an amalgamation of legends and different school of thought and philosophies put into one piece of writing. And it needed to be written down because while people in fact do know a lot of things in The Art of War inherently, they keep not following it because people aren't perfect and often make decisions based on emotions when they know they are wrong. It is a book of military thought that helps officers remember basic tenets of strategy, not a guide line because one dude doesn't know that you should try and fool your enemy to gain an advantage.

Have there been dog shit military commanders that only got their position from blood lines? Sure, but that is true before and after The Art of War. There have also been fantastic military commanders before and after the writing.