Because it's true.
I would say that it's true of any writing, it's merely more apparent in fiction. Beliefs, perspectives and assumptions are in all of us, and they are so pervasive there is barely anything they don't affect. Most important to the issue at hand is that they influence thought, and fiction is a construction made of thought. Contrast that with non-fiction, which is more about objective fact...or poetry, which is usually based on perceptions.
A person might think that a memoir would be a good place to find out about a writer's beliefs, perspectives, and assumptions, but in a memoir the writer takes care to conceal anything they don't like, requiring a sort of literary algebra to reconstruct these things based on what they don't write.