The tools of modern scholarship and science, for starters, help to provide that valid, coherent, and correct reasons for deciding. You can't read it in a vacuum and call that valid or correct reasoning.
I don't think most people who approach the Bible sincerely are deciding these things "just because they want it to be". That is insincerity. That is disingenuousness. That lacks integrity.
Rather, weighing everything as a whole, looking at things like what modern science reveals about the age of the earth, cosmology, evolution, etc, provides a well-researched and well-established scientific fact about the world that sheds some genuine light about the creation story of Genesis. It provides one major example of context for reading it. There are plenty other reasons to understand Genesis as non-literal.
However, to read it literally in denial of that well-established scientific truth is choosing to ignore that "just because they want it to be". That to me is a problem because it
suppresses knowledge in order to maintain a belief. That is not what faith does. That is what fear does.
The choices of books to include into the Bible was a process of decisions made by religious leaders. Those choices reflect their beliefs, while things which did not support those beliefs were excluded. That's not a miracle. That's a deliberate selection process.
However, it's more complex than that. Certain theologies are themselves crafted and shaped taking certain stories or truths and creating a theme, which gets built upon, added to, and tailored to fit the various audiences. In other words, they
evolve over time.
All these things can be seen in the doctrines which got passed on down to you, after having underwent such transformations over the ages. When you sit down and read a passage, you are looking at it through that lens that was passed down to you.
All of that colorizes your understanding of it, because that view passed on to you created the context in which you read it. The "consistent theme" you see, is a creation of views and teachings which shape what you see. Lacking any of that, that "theme" is not nearly so self-evident as the mind makes it appear to be. This is why in part you have different groups seeing different themes, often in contradiction to other group's readings.
As I just pointed out, all of it shaped by these things. You're not reading the Bible in a vacuum. As a great test of this, find someone with zero knowledge of Christian beliefs and teachings and let them read the Bible. What they end up seeing, you would probably call an error, because it doesn't match with your understandings of what and how you are reading it, having been conditioned previously to see it that way.
I always try to make a clear point that I never, ever use the word myth to mean false or a lie. I think I need to add that to my disclaimers in my signature. I always use myth in the technical sense as a type of story that conveys truth through stories. The facts of the story are not the point of a myth. The message is the point. Adam and Eve are,
in my educated opinion, fictional characters, whose purpose is to represent all of us. They symbolize the truth of the human condition, through imaginary settings and storyline.
A good mythology is timeless in nature. I love the myth of the Garden of Eden, as it symbolically speaks truth - non-literally. There reaches a point where one can see the Truth without needing to literalize things into concrete historical facts. At that point, then we don't have to hide from knowledge which challenges that literalization. Such as denying evolution.
Again, I do not dismiss something because it is considered a mythology. On the contrary, I try to understand what it's trying to communicate through its imaginary characters and settings. Mythology is simply a category of types of storytelling.
For an understanding of what those like me mean when we refer to the Bible as containing mythologies, look into Joseph Cambell's work for starters.
Joseph Campbell and the Myth of the Hero’s Journey