If Sikhi, is the true religion. How come it was created/revealed 300 years ago, and not at the beginning of time?

Which religion was created at the beginning of time? Only god existed at the beginning of time and while Sikhi accepts that spiritual progress was open to human beings for many years prior to the arrival of Guru Nanak Dev Ji, only after Guru Nanak Dev Ji came to this world did the pure merger (in Sach Khand) between our soul and God become possible for all, as the Name of God and the Scriptures of God (gurbani) were revealed.

Related posts:
* Does the Guru Granth Sahib claim to be a revelation direct from God?
* Did anyone get to God or Sach Khand before Guru Nanak?
Humans have existed for tens of thousands of years. They existed in North America and Asia with established cultures 10 000 years ago. That is much before any modern religion. What must have become of those poor souls?
If we accept the Islamic approach, we have one shot at life. If we’re a success we go to Paradise and if not, we go to Dozak/Hell. But that leaves a lot of questions.
Is God perfectly Just and fair? If he is, then why are some people born Muslims and other born in other religions? Those people born in other religions will have a much greater struggle and test to become Muslims than those born in Muslim homes. But at the end, the reward is the same: Paradise. So why do some people have to work so much harder for the same result that others earn much more easily? It’s not fair.
What about babies that are still born and die in the womb? What about babies that die shortly after birth? What purpose did their life have? Why would God create them and let them die so soon? Their life didn’t have any real ‘meaning’ and they faced no real tests nor did they have the opportunity to embrace a faith.
Finally, going back to the question, what about the millions of people that came before Islam or the Semitic religions or even now, the millions of people that will never be exposed to Islam? Why are they treated so unfairly? What will become of them?
Sikhi has the answer to all these questions. The soul proceeds on a path through many lives. When it is ready, it finds Sikhi and is able to be liberated through meditation on Naam. If someone is good but hasn’t received Naam, then maybe in their next life they will be given the opportunity. No one is consigned to eternal Hell. Those souls that came before Sikhi were on a path preparing themselves to receive the divine revelation. As they become ready, they will now get the opportunity to learn of Sikhi and be saved.
As for your questions about Guru Nanak: Mohammed was not born a “Muslim” and Christ was not born a Christian. Guru Nanak was born in a Hindu household but did not accept any Hindu ritual even in childhood including the ‘sacred thread’, etc. Guru Nanak tells how he was summoned to God’s court and assigned the duty of spreading Naam and being God’s messenger. The ang (page) with one example of such a verse is 150. Guru Nanak then took this divine message further than any other “Prophet”. He traveled as far as China, Sri Lanka. Russia and Turkey. He may have traveled farther but historical proofs exist for the countries mentioned.
Guru Nanak’s companion, Mardana was indeed a Sikh. The story of how he became a Sikh and the conditions he accepted are also recorded. In fact, when he died, he was cremated by Guru Nanak. If he were Muslim, he would have been buried.
Guru Nanak coined the phrase “Sikh” and used it to describe those on the Path of “Gurmat”.

As for revealing the “holy book”, Guru Nanak’s verses are the “holy book”. As he was receiving them, he wrote them in his own hand. What further proof does one need?