The Mundaka Upanishad Mantra you quoted is -
नायमात्मा प्रवचनेन लभ्यो न मेधया न बहुना श्रुतेन ।
तेन लभ्यस्तस्यैष आत्मा विवृणुते तनूं स्वाम् ॥ ३ ॥
nāyamātmā pravacanena labhyo na medhayā na bahunā śrutena |
yamevaiṣa vṛṇute tena labhyastasyaiṣa ātmā vivṛṇute tanūṃ svām || 3
This Atman cannot be attained by dint of study or intelligence or
much hearing—whom he wishes to attain—by that it can be attained. To
him this Atman reveals its true nature.
Shankara’s Commentary: -:
This atman which has been explained and whose realisation is the
highest object of human desire cannot be attained by means of much
study of the Vedas and the Sastras. Similarly not by intelligence,
i.e., by a retentive memory of the purport of writings; nor by much
heard, i.e., by much hearing. By what then could the Atman be attained
is explained. The Paramatman whom this knower wishes to attain, by
that seeking alone can that Brahman be attained;
The drift is the wish for the realisation of the Atman after
renouncing all others is alone the means to the attainment of the
The whole purpose of the below verse is to tell us the way or the means by which Atman i.e. Brahman of Vedanata can be known. And the point is the atman can not be attained only by plenty of reading of Vedas and Shastras . Nor mere by by too much intelligence and hearing. But by desire of the one who wants to know him. The verse here is talking about the wish or desire of the person who wants to know this Atman Or Brahman. This upanishadic verse is about Gyan-Marga or about the path of knowledge .i.e. vidya.
Now lets talk about the Bhagvatam Verse under which the purport is given which is quoting the above verse . Note that again it is a Purport: Not the actual Translation or the actual meaning.
श्रुतेन तपसा वा किं वचोभिश्चित्तवृत्तिभिः ।
बुद्ध्या वा किं
निपुणया बलेनेन्द्रियराधसा ॥SB 4.31.11॥
Without devotional service, what is the meaning of severe austerities,
the process of hearing, the power of speech, the power of mental
speculation, elevated intelligence, strength, and the power of the
This is saying of Narada , In which he is advising to pracetas about ineffectiveness of the ways other than a way by which this brahman can be attained i.e. Bhakti. So basically this verse is centered towards giving importance to Bhakti Marga , other than Gyan or Karma Marga. Again the purpose of the verse is to highlight Bhakti. And this is nor saying of krishna nor of vyasa. The verse is the part of story and only to be interpreted in that context of that story.
Now these two ways are totally different and are the independent ways to attain Atman Or Brahman. So are the two verses ,one is from Gyana portion of vedas and one is from purana which emphasis the bhakti. I think the connection shown in the puurport of the above verse is not at all valid , we cant deny laws of physics by laws of chemistry , in the same way we cant compare both these texts , as vedas and upanishads are the highest authoritative texts of hinduism on Brahman .
jnanamarg or the path of knowledge. It said to be ideal for those who are intellectually curious and want to pursue God in an intellectual, intuitive and scholarly way, through the study of scriptures, practice of yoga and meditation and discipline of the mind and the body. The people who follow this path are called Jnana Yogis or jnana margies . This is considered to be a difficult path
The one other path is the path of devotion, which is said to be ideal for those who have neither the inclination to pursue the path of knowledge through the observation of austerities and practice of spiritual disciplines, nor the path of actions through detachment, surrender and sense of sacrifice.
About your enquiry - why many people dedicate their lives in the studying of the Vedas If one simply needs to understand the Supreme Personality of Godhead.
The simple answer is for the people who prefer the path of gyana or the one who are gyan-yogies the liberation or the realization of Brahman /Atman comes from reading the shastras or scriptures and by following the path shown by upanishads , so reading of vedas and shastras is important for gyan-yogies to know the Brahman in the same way for the people who is following path of devotion ,Bhakti is required.