Why is it so difficult to believe in a deity (or theity as I like to say)? It's the evidence, and lack thereof.
One of the things I like to point to is the origins of Earth and life and compare it with the Old Testament. The birth of our planet is a remarkable story, and the origin of life, though not completely understood yet, is just as amazing.
If you were to take ANY of the correspondence with supernatural beings in the bible (NT included), how come none of them refer to evolution? When "Moses" was chatting with "Yahweh," why wouldn't the big bully tell it like it is? Why would he lie to Moses and say he made the Earth and universe and humans in a few days? In the NT, why didn't Jesus know about germs and viruses, the micro organisms that were killing his flock and children to the tune of one in every two kids die of disease? If you believe the bible, he actually said we don't need to wash our hands before eating because nothing can hurt us.
Let's take this from another angle: If there is a god, and you believe (rightfully so) in evolution, why would he create life as a single-celled organism that would take billions of years to become human? If he were truly omnipotent, wouldn't he just make humans "in his own image" like it says in the bible? Just think about this for a minute. If humans were created in a god's image, why would he start with a process that would split into countless species and need to evolve over billions of years to get to "his image?" And for that matter, we know the universe is nearly 14 billion years old, why would he wait nearly 10 billion years after that to make the planet his children would live on? What was he waiting for?
And since you believe in evolution, then you know humans (in our current evolved state of Homo sapiens) have been walking on this planet for about a quarter of a million years. Why did the Abrahamic god wait about 245K years before "revealing" himself to us, letting us suffer for a couple hundred millennia? That's too painful and ridiculous for believers to accept, so the goat-herders believe humans first populated Earth in that Genesis week, otherwise they couldn't justify their god.
This is why/proof the desert-dwellers made up Genesis, because they just didn't understand how things worked or evolved. It's the ultimate god of the gaps fallacy, because once you realize evolution is true, and the universe is billions upon billions of years old, you know god can't exist. It makes no sense when you know the facts.