Based on the teachings in Chapter 20, why is self-effort alone insufficient for understanding profound scriptures like the Upanishads?

Chapter 20 explains that scriptures like the Upanishads, which contain the essence of the Vedas, are incredibly difficult to grasp without the grace of Hari (God) and a Guru. The text explicitly states that a scholar who believes they can comprehend these texts through their own efforts will fail, even at the end of an era. This is because the hidden truth does not reveal itself without Guru's grace, and the path is difficult at every step. Only for one who is humble, detached, and attached to the Guru's feet does the hidden meaning open up automatically.