All praise is due to Allah, and peace and blessings be upon the Messenger of Allah.
Belief in the existence of Allah is part of the natural disposition (fitrah) that Allah has placed within human beings. This is affirmed by both the Qur'an and the Sunnah. Allah says:
"The natural disposition of Allah upon which He has created mankind."
(Qur'an 30:30)
However, this fitrah may become weakened or distorted because of one's environment, upbringing, misconceptions, or the influence of devils among humans and jinn. As mentioned in the sacred hadith (Hadith Qudsi), Allah created His servants upon the upright natural disposition, but the devils turned them away from their religion.
Therefore, the existence of individuals or groups who deny Allah or associate partners with Him does not negate the existence of the fitrah. Rather, it shows that the fitrah can become obscured or corrupted by external influences, just as a person's natural moral character can be corrupted even though it is originally part of human nature.
Moreover, throughout history, those who have denied the existence of Allah have represented only a very small minority compared to the vast majority of humanity, who have believed in a Creator, even if they differed regarding how He should be worshipped or fell into polytheism.
As for claims about certain primitive tribes, it has not been conclusively established that they deny the existence of a Creator. Often, the intended meaning is that they do not follow a specific religion or lack a clear concept of Allah, which is different from explicitly denying the existence of a Creator.
The existence of a few individuals or tribes who do not outwardly believe in Allah does not invalidate the innate nature of faith. The fitrah may become distorted or concealed because of various influences, and such cases remain exceptional. They do not change the reality affirmed by Divine Revelation and reflected in the beliefs of the overwhelming majority of humanity throughout history.
And Allah knows best.