Why is it considered so difficult to understand the Upanishads without the help of a Guru?

According to the narrative in Chapter 20, the Upanishads contain the very essence of the four Vedas and are filled with profound, hidden meanings. The text explicitly states that trying to comprehend them through one's own scholarly efforts is a futile task, even if one tries 'at the end of an era.' The path is described as 'difficult at every step' without a Guru's grace. However, for a devotee who is attached to the Guru's feet, the text promises that 'the hidden meaning opens up automatically,' removing all difficulties and obstructions of the intellect. This emphasizes that spiritual insight is a matter of grace, not just intellectual power.