I can see why teachers would tell a student, “write what you know” in the sense that doing all the research can hinder the learning of telling a good story. Just keep the subject simple and work on the art and craft of writing. Scaffolding the learning as it were. Not that writing about your life doesn’t require a little research, because it does. One other aspect is to question what you do know or think you know. Turn it upside down and inside out. Re-envision your experiences. From what other lens can you see the same experience? As William Blake observed, the same world to one man is a heaven and to another a hell.
I started to think about “writing what you know” from a different angle. Instead of what you know as in life experiences to what you can know through research. Doing research led to what does it mean and in coming to terms with the fact that sometimes there is no pat answer or “moral of the story” I freed myself in a different way. I would hasten to add, there is sea of difference between an answer and a revelation.
This of course led to escape the idea of the physical story altogether and consider the emotional story, that emotional quest the protagonist embarks upon and what questions it raises.