Sun Tzu didn't write a textbook for modern audiences. In modern textbooks, the authors include examples, illustrations, exercises, and practice drills to help students master the ideas. Sun Tzu's work lacks all these features.
When you start reading the Art of War, you may notice how much of the work is spent simply defining terms. This process begins on the first page and continues on almost every page. As we read, we cannot keep track of this multitude of definitions. As one definition follows another, the important details of their critical elements are easy to overlook.
Illustrations demonstrating the connections among these ideas would be very helpful, and, surprisingly enough, his work does include descriptions of such illustrations, but they are written in the scientific tradition of his era. Modern readers have no idea how this culture mapped ideas, so we cannot make the connections that readers in his era did naturally.
As the work goes on, Sun Tzu uses his specialized vocabulary to express very sophisticated ideas. Most of these ideas cannot be easily expressed without that vocabulary. However, since readers do not master the fine distinctions of that vocabulary, they cannot understand the more refined points that are being made.
If Sun Tzu had written in modern times, he would have explained these ideas in more detail. He would have included examples, exercises, and practice drills to clarify his concepts.
Unfortunately, in his era, people learned from living masters, not from books. On a practical level, writing and duplicating books was too expensive and time-consuming to include a wealth of examples or details. The onus was placed on the reader to study the work rather than on the writer to explain every idea in detail. There is a 2,500-year gap between Sun Tzu's expectations and readers' expectations today.