Hermeneutics. If you make Cain mythological then why not make Abraham a myth also. On what literary nail do you hang your mythological label and your literal historical label? Because it is an old story? It's really not so old that it is impossible to be a real history. Because you think it was passed down orally until Moses and can't possibly have been preserved accurately? Why is it so hard to take it as literal historical as well as Abraham's story?
When you read genesis 1-11, with all its play on names, play on words, extreme situations, and spanning thousands of years with relatively little information.
Then read through the entire rest of the Torah and see does it carry the same weight?
Just think about a few key things.
1. The earth is just water, not frozen, even though a sun does not yet exist.
2. A flat circle shaped earth is protected from space water by a invisible dome.
3. There is a tree that a couple eats from that sustains their immortal life. After all why would immortal beings need a tree that grants immortality? So the tale is two humans that are sustained by a magic fruit that grants life.
4. That couple is made up of a mud golem who was breathed to life and his partner which was made from his side.
5. All of this occurred on the sixth day. Because it says on the sixth day God crested all animals on land and made humans, both male and female. So on the sixth day God gathered mud, breathed on it, and then called it human ( Adam means human ) and saw that it was not good ( so it was not perfect ) for man to be alone so he called all animals forth on the entire planet and Adam named each one of them ( millions of animals ) and none of them was satisfactory to be a partner. So he put him into a deep sleep ( much to be said on the meaning of that ) and made a woman from his side and called her life ( eve means life)
In a nutshell this is the first few chapters of genesis.
A water planet without any sun remains unfrozen. Over six days God generates everything in existence and on the sixth day he made a man from mud and a woman from the man’s side. They lived in a garden eating fruit which included one that sustained their life. These mortal beings were kept immortal through the magical fruit. One day a talking snake tricks them. They get cast out and have a bunch of sons and daughters. Eventually they get to Cain and Abel. Cain murders Abel and then flees as a wanderer banished from Eden. Before leaving he’s given a tattoo that warns anyone who kills him they will meet a worse fate. Then the couple is given another son named Seth. Cain and one of sisters have kids together and within there genealogy no ages are given and within Seth’s , who also marries a sister, they are given a genealogy full of extraordinary long lives. Roughly a thousand years after a Seth’s birth is a child named Noah who build a giant ark that houses 2-7 specimens of every known species while the dome up above the earth opens allowing space water to pour down while water below earth is erupting forth flooding and killing everyone not on the boat. ( how salt water fish survived the fresh water saturation is unknown). They all survive and repopulate the earth , somehow the giants also survived because it does mention these special potentially half human half angel offsprings before and after the flood. Lots of more time goes by and all of mankind join together in harmony and begin to build a tower that can reach heaven ( how was a building going to reach heaven we don’t really know - among a literal interpretation anyways. God , who can be everywhere all the time decides to come down ( because apparently heaven is literally up above us beyond the clouds ) to earth and sees what’s going on and so he destroys the tower and causes all of humanity to somehow learn different languages and they can no longer talk to one another and it’s at this time that Pangea begins to be broken apart as nations drift across the ocean.
That’s a summary of eleven chapters.
Now make summaries of the next 11 chapters and several of them are , people are still in the desert people are still in the desert.
So I believe 100% in God and in the bible as being 100% true. But that does not mean I believe it’s 100% literal.