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

Chapter 20 powerfully illustrates the necessity of a Guru's grace for true spiritual understanding. The text explains that the Upanishads contain the essence of the Vedas and are full of deep, hidden meanings. It explicitly states that a scholar who believes they can comprehend these texts through their own efforts alone will fail, even "at the end of an era." As mentioned in Chapter 20, the path is "difficult at every step" without a Guru's guidance. Conversely, for a devotee attached to the Guru's feet, there is "not even a bit of difficulty," and the hidden meanings open up automatically. This highlights that spiritual knowledge requires humility and surrender.