It is more about the reason for our personal conviction..
Why is it that people all over the world have become Muslims and Christians?
Why is it that Sikhism and Hinduism haven't become internationally acknowledged as being "the truth"?
They don't actively proselytize, unlike Christians and Muslims.
..depends what you are looking for, I would say. Mankind is tribal, and "believe" for reasons other than acknowledgement of truth, often expressing dislike for change etc.
Tradition.
Some people are atheists, and prefer secular values as they value the culture of the country they come from, to the extent that they cannot see that their civilization is seriously declining.
We do observe civilization declining. Just not convinced religion is the silver bullet. Often, it seems, it is mixed in with other causes of the decline, in fact, like the tribalism you mentioned. It seems to be more divisive than unifying, in general.