When we ask why there isn’t a certain something or why there is something that is a certain way, what we neglect to remember is that everything is a work in progress. Things aren’t something — they are always changing and becoming something new.
So let me answer by throwing some light on what we were and the changes we are going through as a world.
On the whole, there is not much variation among human beings. We all pretty much look alike, we behave in similar ways. An smart enough outside observer can perhaps even predict how any one of us will react to situations involving danger, hunger, loss, fear etc. We are, on the whole, fairly predictable.
Perhaps this is why, despite the fact that early human tribes travelled a lot and were separated from each other by continents (and even seas eventually), our reactions to our surroundings were pretty similar. We felt afraid, grateful, happy, sad, and curious. Out of these general feelings emerged a way of living — a set of guidelines that implied that surviving together was better than dying alone. This emergent set of guidelines was morality. Mixed with tribal storytelling, it turned into religion.
When different tribes met, they either fought, or looked at each other with a flicker of recognition.
“These other guys are not that different from us,” they thought.
“Perhaps we should talk,” someone said.
“Hey, that guy looks like he is about to die,” someone pointed out. “We should get him medical help.”
“Maybe you are right,” said his friend. “But we don’t know what their bodies are like. Maybe they have four hearts. Maybe our herbs won’t work on them.”
“He looks like us,” said the first guy. “Let’s at least try”
So they tried. It worked. Turns out, the other tribe was just like them. Their vague feeling, that this other tribe was like them is confirmed. Human recognises human. Some are confused. Some are happy. Some say this is wrong — their “ways” are so different.
“They tell strange stories about animal spirits we have never heard of. They worship that other tree, which we consider evil. They actually eat this animal that we don’t touch because my grandfather was killed by one.”
By and large however, tribes mixed. It took time, fighting, and plenty of storytelling at nighttime around fires. The storytellers, who were holy men in all tribes, will be known as religious figures later. They remix the stories of different tribes and come up with unique mythologies. These mythologies change over time as well, as more and more tribes combine.
To the followers of any one mythology, it always looks like the stories they have heard are set in stone and unchanging. But that is never the case. Stories are hardly ever the same century from century. New versions emerge, become mainstream (depending on social factors) and become the default value for the next generation. The assimilation of Jagannath into the Hindu pantheon is one example. The assimilation of Buddhism is another. Christianity has also assimilated tons of pagan rituals and practices. There are also examples of how Indian Islam assimilated elements of Hindu culture.
After each assimilation however, comes denial. The generations that follow say that the shape their religion is in right now is “eternal” (as opposed to every other religion of course, which are passing fads).
There came a time when most of the human world was dominated by similar gods and deities. They wielded similar powers, occupied similar domains, and even had similar origin stories. Zeus and Indra are a case in point. It was a time when almost all humanity could be called pagan. Tribes met but did not fight over religious beliefs. Instead, they pointed at each other’s gods and said, “Hey. Nice. We have one just like that. We call him something else of course, but he does pretty much the same things. Can I sit and watch while you pray?”
Encounters such as this were common in the ancient world. People understood what was happening in a general sense. They knew that people were just people and that the religious impulse was the same everywhere even though it appeared to take different shapes.
But then came monotheism. Its gods were different from those already existing in other cultures. Monothesists were exclusivist by nature. When they pointed at the strange gods (from their perspective) of another culture, they did not express recognition and understanding. They said, “This is a false god. There can’t be so many gods. There can only be one god. We have that one god. You must abandon your gods and join us and worship that one god.”
Needless to say, some people said, “What the F!”
To this day, the two main exclusivist monotheisms of the world — Christianity and Islam are causing people to say “What the F!” with their authoritarian claims and their desire to convert everyone into ONE religion. Christianity’s zeal takes the form of conversion. Islam gets more warlike and violent in its enthusiasm. Both these religions have split away from Judaism, which first presented the idea of ONE unforgiving god, but their respective mythologies have evolved to become so different from each other that they look like different faiths.
Thing is, religions merge. They have done so since the beginning of civilisation. Their merging is not about to stop. The religious impulse is strong and people will always believe. But the way for a religion to prosper and grow is, in all likelihood, not going to be the complete overthrow of others. Arthur C. Clarke, in his science fiction novel, suggested a future religion called Chrislam — a merging of the monotheisms. It’s still nutty, but now they are trying to communicate with aliens.
I think there will one day be one religion. But it seems unlikely that it will be one that is in existence right now. It may in fact not be a religion at all. The whole history of humankind has been the process of this one religion. Through hits and misses, through trials and errors, we have been moving towards it.
 Vijayendra Mohanty’s answer to Why do we like stories, and why is it that we always have the urge to know how a story ends?
 Jagannath – Wikipedia
 Studies in the Buddhistic Culture of India During the 7th and 8th Centuries A.D.
 “The Influence of the Mystery Religions on Christianity”
 Buy God Against the Gods: The History of the War Between Monotheism and Polytheism Book Online at Low Prices in India
 The Hammer of God (Clarke novel) – Wikipedia