If the scriptures are allegorical or symbolic, why did the authors choose to write in this complicated way? Why not just spell it out like it is done in the contemporary books? I would think the saints would want to be compassionate to trees and save lots of paper that would be used on interpretations, and alas, misinterpretations!
You’re essentially asking, "Why do great masters do what they do?" I would say that the answer is, "Because God told them to," — and they’ve learned to listen. I can’t give you a more definitive answer than that, but here are a few thoughts that I hope will feel reasonable.
First, not all scriptures are allegorical. Yes, Jesus Christ told parables—“Those who have ears to hear, let them hear”—but he also gave many direct, literal teachings. So it is with many Hindu and Buddhist scriptures—allegories, yes, but also many direct teachings. The Bhagavad Gita, for example, takes place within an epic allegory, but the teachings themselves are very direct (though not always easy to understand!).
Yet other scriptures are indeed highly allegorical, and beg for explanations. Two prominent examples are the ancient Vedas of India and the Rubaiyat of Omar Khayyam (try to understand that without a commentary! I suggest The Rubaiyat of Omar Khayyam Explained).
One reason for allegory is that it attracts interest more readily than direct teachings. People can relate to stories.
Another reason is that allegory helps spiritual teachings survive during times of relatively dark consciousness, because stories are more visceral than direct teachings, and thus more memorable. Even in times of higher consciousness, there are some people of relatively low consciousness who cannot relate to direct teachings, but can relate to stories. A story might cause a light to go on in the hidden recesses of their minds, resulting in some helpful (even if only subconscious) understanding.
I don’t doubt that, as the world moves into an age of higher understanding, many of today’s seemingly “spelled-out” teachings are going to seem needlessly complicated or vague. People in such an age may well ask the same question you’re asking: “Why were those authors so indirect?” And the answer will be that people in our current age simply aren’t ready for the level of directness appropriate in a higher age. We’re getting what we can handle today.
And will there be misinterpretations? Always. Even the direct teachings that are so readily available today can be—and often are—misinterpreted. Such is the human condition. Those who have ears to hear, let them hear.