According to Chapter 20, why is a Guru's grace considered essential for understanding profound scriptures like the Upanishads?

Chapter 20 strongly emphasizes the necessity of a Guru for comprehending deep spiritual texts. The text states that the Upanishads contain the essence of the Vedas and are very difficult to grasp without the grace of a Guru. It warns that even a great scholar who tries to comprehend these scriptures through their own efforts will fail, as the hidden truth is difficult to find without a Guru's guidance. As mentioned in Chapter 20, for a devotee attached to the Guru's feet, the path becomes easy, and the hidden meanings of the science of Self-knowledge open up automatically.