If God exists, why did he allowed humans to evolve for billions of years?

If God existed then yes, God could have created the entire universe in an instant, and the idea of a "six day creation" makes no sense, on the other hand, given what we know about the timeline from the start of the universe until humans evolved, there is no way to match any creation story realistically.

Either God exists and is intentionally being deceptive (i.e. lying) or God does not exist and everything in the universe formed naturally.  The evidence is strongly infavor of the universe forming naturally.

Evolution and the evidence for it constitute a threat to those who try to literally believe what the Bible says about creation.  It provides an alternative pathway that doesn't require God at all, and that frightens them, the cowering little sheep that they are.

Humans didnt evolve over billions of years. God created humans during the sixth creative epoch of Genesis.

Yes, He did use biological evolution to create our physical/biological structure. But, we are more than just intelligent animals. God directly made us fully human when He gave us a spiritual nature and an immortal soul, characteristics that could never have been produced by any biological process.

God has billions of Sons and these universes are their playground/Laboratory. With the Father's permission, they were attempting to build their own Heaven. They caused this messed up world from the start by not following instructions. Humans are the product of God's Sons.  

False assumption, without evidence, that humans evolved, so the question is moot.

God actually creates everything over a period of 6000 years ago.Sorry to burst your bubble but the billions of years you think happened never existed. There was no evolution. God designed all  the living creatures to reproduce after their own kind. Fish never walked into the land. God created the earth as it is now, all the animals are exactly how God designed them to do and look like.

Because the world doesn't revolve around us.

I think you're mixing up science scaling and Biblical scaling.  

In the Bible, it may have been instantly or only a matter of days. 

That's the joke about science's exaggerated scaling - similar to distances.  

"You are my sunshine, don't be so far away."

Science says the sun is 92 (or 64) million miles away from the Earth.

In the Biblical model, things are much closer together.  

Religion often fails to consider the realities of science. The reason of course is because when this Abrahamic religion began, science was unknown, disregarded or perhaps even heresy. You followed the teachings of the holy men or some form of the Lord's vengeance would be exacted upon you.

Religion was begun as a means to understand our world. The mortal beings couldn't conceive how such an event came about, so therefore the God thingy did it.