Because most people write in unconscious imitation of another writer.
Take recently deceased Elmore Leonard. He had a lot of rules for writing, such as never use anything but said. He was a great believer in simple writing, with great dialogue. His goal was cut anything that a reader my skip when he got bored. If it wasn't action, he didn't want it.
That is a style.
Or take Hemingway. Hemingway hated descriptive writing. He thought it was lazy. He once wrote a novel, where the only descriptive sentence of the woman in it was "She took off her hat and set it on the table." Other than telling you she had a hat on he didn't say a peep about her looks, how she was dressed, or a play by play of her facial motions.
That is a style
Bret Easton Ellis saturates his writing with the minutiae of consumerism, explicit detail of fonts and thread counts, till you feel the ennui of his characters. He floods you with detail, a glut of facts.
That is a style.
Hunter S Thompson wrote of his own life in an overblown style a mixture of Bukowski and an exaggeration of the minor incident into the epic. He's the reason so much rock writing sucks eggs.
yet that is a style.
How do you get your own style? You mix and match other people's style like a Chinese menu. You rewrite your own work, because when you read it, it sounds like you're imitating Steinbeck or Hemingway, or Tolkien. Then you rewrite again, because you realize you lifted too much from another writer this time.
You read good writing and then you write, till your own style emerges, a blend of a hundred authors, rather than a teenager's hero worship of one.
It is no different than art or music, where every artist gets his start imitating others till he starts to take his own steps which aren't just "Ok I'm going to sing this like Neil Young." or "let's do another Van Gogh rip off."