Anyway, that was kind of the point of these so-called holy books and stories. Humans, by nature, are curious. They seek answers. These tales were a way of providing comfort to people who had deep existential questions that, at the time, could not be answered by science or logic. The problem is when people take these tales literally still today. The Catholic Church, for instance, has stated that there are no problems with evolution or the big bang theory, and yet many followers (and teachers too) still hold on to the Adam and Eve and seven days nonsense as fact - ignoring the purpose of these tales in their original context.
That aside, I think the "big five" as you described them have some degree of legitimacy (or something like that) due partly to their age. Or more specifically, all the extra that came along with their longevity. They've contributed to art, culture, history, philosophy, etc. The new guys don't have that backstory that contributes to a perceived legitimacy. If any of the big five started today in a vacuum, like the other new religions of today are, I'd think people would look at them like comic books lol