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

Chapter 20 powerfully argues that a Guru's grace is essential for true scriptural understanding. The text states that the Upanishads hold the essence of the four Vedas and are very difficult to grasp without the grace of Hari (God) and the Guru. It warns that a scholar's attempt to comprehend them through personal effort alone is futile, as 'the hidden truth does not come to hand.' In contrast, Chapter 20 assures that for one who is devoted to the Guru's feet, the hidden meanings open up automatically. This 'science of Self-knowledge' is described as a weapon against the cycle of rebirth, accessible only to the humble.