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

Chapter 20 of the Sai Satcharitra strongly emphasizes that a Guru's grace is indispensable for comprehending profound spiritual texts. The text states that the Upanishads contain the essence of the four Vedas and are incredibly difficult to grasp without the grace of both God and the Guru. It warns that a scholar who tries to understand them through their own efforts alone will fail, as the path is difficult at every step. However, for a devotee who is attached to the Guru's feet, the hidden meanings of these scriptures open up automatically. This science of Self-knowledge is only accessible to the humble who have a Guru's guidance.