Actors have known this for decades: self-expression isn’t only a stage problem. It’s a life problem. Most people fail to express themselves on an hourly basis. Being good at expressing yourself is unnatural. Having clarity of what “yourself” even is is unnatural. The truth is that we’re all making comments, jokes, deciding what’s important and what not using old programming in our brains… programming that was given to us by our childhood and our education. Very few people can consistently have the luxury of being/ability to be creative with that old programming, and even those that can often have to plan ahead of time/rigidly control the environment in order to achieve a creative result.
The exact same problem exists with writing. In fact, this problem seems to exist across all fields: science, for example, is filled with people who have never done a groundbreaking study, presented a new idea, or solved an unsolved problem. These people and their jobs are so common that the education system orients itself to teach to them rather than anyone else. In the same way, an education in literature focused on the more likely traits you’ll need to get a job: hitting deadlines, following the expected story structure, etc etc.
Having confined ourselves to a tiny little box, can we really be surprised that we’re so easy to imitate?