The answer to your question from an incident that is found in the Srimad Bhagavatham Mahapuran, which is likened to the mature fruit of the desire tree of Vedic literature. This happened at the end of Dvapara yoga, approximately 5000 years ago.
Excerpt from Srimad Bhagavatham Chapter 4
Once upon a time he [Vyāsadeva], as the sun rose, took his morning
ablution in the waters of the Sarasvatī and sat alone to concentrate.
The great sage Vyāsadeva saw anomalies in the duties of the
millennium. This happens on the earth in different ages, due to the
unseen force of time.
The great sage, who was fully equipped in knowledge, could see through
his transcendental vision the deterioration of everything material due
to the influence of the age. He could also see that the faithless
people in general would be reduced in duration of life and would be
impatient due to lack of goodness. Thus he contemplated for the
welfare of men in all statuses and orders of life.
He saw that the sacrifices mentioned in the Vedas were means by which
the people’s occupations could be purified. And to simplify the
process he divided the one Veda into four, in order to expand them
among men. The four divisions of the original sources of knowledge
[the Vedas] were made separately. But the historical facts and
authentic stories mentioned in the Purāṇas are called the fifth Veda.
Thus the great sage Vyāsadeva, who is very kind to the ignorant masses, edited the Vedas so they might be assimilated by less
intellectual men. Out of compassion, the great sage thought it wise
that this would enable men to achieve the ultimate goal of life. Thus
he compiled the great historical narration called the Mahābhārata for
women, laborers and friends of the twice-born.
The Vedas are esoteric literature which can't be understood by men of less intelligence. As foreseen by Vedvyasa in his meditation, men in Kali yoga are less intelligent and less advanced in spirituality. This is also confirmed in Srimad Bhagavatham else where.
O learned one, in this iron Age of Kali men almost always have but
short lives. They are quarrelsome, lazy, misguided, unlucky and, above
all, always disturbed.
Srimad Bhagavatham 1.1.10
But the reality seems worse than the prediction. Most Hindus don't attempt to learn the exposition of Dharma given in Mahabharatha or Puranas. The Sanskrit literacy rate is very low. Nobody reads the translations of these books either.
The Vedas explain abstract concepts, whereas the Puranas and Mahabharatha explain those concepts in practice with examples. For example, we can understand how to live a Vedic lifestyle from the characters of Valmiki Ramayana.
Prasthyana Trayi refers to Nyaya Prasthana, Shruti Prasthana and Smriti Prathana. Nyaya Prasthana is Brahma Sutras, Shruti Prasthana is the principal upanishads, Smriti Prasthana is Bhagavad Gita. Shruti means Vedas, Smriti means Mahabharata and Puranas. Since, it is not possible to interpret the Vedas in this age, due to lack of scholarship, we can take shelter of the Smriti such as Bhagavad Gita and Srimad Bhagavatham, etc. If you just want a single primary book of reference, Bhagavad Gita is sufficient as the golden reference for Sanatana Dharma or the Vedic Culture.
In Gita Mahatmya written by Shankaracharya it is said
ekam sastram devaki-putra-gitam
eko devo devaki-putra eva
eko mantras tasya namani yani
karmapy ekam tasya devasya seva
There is one scripture only, one common scripture for the whole world, for the people of the whole world, and that is this
Bhagavad-gita. There is one God for the whole world, is Sri Krsna. And
one hymn, is to chant His name. There is one ritual, to worship the
Supreme Personality of Godhead, Sri Krishna.
Vedic literature constitutes a huge library, which can't be mastered even in many lifetimes. I have been told by a Vedic Scholar, that each Veda takes 14 years for a man with a normal IQ to master it. So the four Vedas would take 56 years only to learn, what to talk of understanding the meaning and applying them in life. In the current situation, we can't even invest that extra two years for a masters degree in a subject.
It takes one year to do a systematic study of Bhagavad Gita. Let's say one does not even want to invest this time in learning Bhagavad Gita. The sages in Naimisharanya ask this specific question to Suta Goswami, who is the professor-in-charge of the Puranas, whose answer forms the basis for Srimad Bhagavatham.
Srimad Bhagavatham 1.1.11
There are many varieties of scriptures, and in all of them there are many prescribed duties, which can be learned only after many years of
study in their various divisions. Therefore, O sage, please select the
essence of all these scriptures and explain it for the good of all
living beings, that by such instruction their hearts may be fully
satisfied.
First we said, there are many Vedic literature, now with the answer of Suta Goswami, it becomes clear that there is not even need for even one textbook. The entire Vedas condensed to one single verse. Here you go...
Srimad Bhagavatham 1.2.6
The supreme occupation [dharma] for all humanity is that by which men can attain to loving devotional service unto the transcendent
Lord, Sri Krishna. Such devotional service must be unmotivated and
uninterrupted to completely satisfy the self.
What is the practical demonstration of this principle? We can see that from the way Arjuna and his brothers, Pandavas lived, tolerating the hardships and happiness for the satisfaction of Lord Krishna.
Again the same thing has been confirmed by Shankaracharya in Moha Mudgara Stotra, Bhaja govindam.
भज गोविन्दं भज गोविन्दं
गोविन्दं भज मूढमते ।
सम्प्राप्ते सन्निहिते काले
नहि नहि रक्षति डुकृङ्करणे ॥
Worship Govinda, worship Govinda,
Worship Govinda, oh deluded mind!
At the time of your death,
Rules of grammar will not save you.
So, the essential point of Vedas is to remember Narayana at the time of death. Everything else in Vedas is external details such as ritualistic activities for fruitive workers who desire elevation to heavenly planets and monistic knowledge for the scholarly mental speculators who can't accept the transcendental form of the lord and who can't agree to do humble service of the Lord due to lack of purity of consciousness.
As confirmed by Lord Sri Krishna in Bhagavad Gita, 8.5
anta-kale ca mam eva
smaran muktva kalevaram
yah prayati sa mad-bhavam
yati nasty atra samsayah
And whoever, at the time of death, quits his body, remembering Me alone, at once attains My nature. Of this there is no doubt.
