Why do we need classes to learn math or science? It isn't just memorizing facts. It takes time to master new concepts.
In The Art of War, the problem starts by
reading Sun Tzu like modern nonfiction. There are a number of serious conceptual differences in what we expect and what it is.
Sun Tzu explains his concepts, but not like a modern textbook but like a scientific work with very specific definitions of concepts. Much of it is expressed in the rich scientific tradition of his era that modern readers cannot comprehend.
Because of the depth of meaning in each word, every line in The Art of War has a wealth of useful applications. This, however, does not mean that you can make up your own interpretation. Its concepts and their connections are rigorous. The only real secret to Sun Tzu is that it takes time and effort to learn and master his system.