I am not a doctor and am instead just a clown on a computer, so, like, don't take me seriously. BUT Can you imagine other things in your life? Like, do you create pictures in your head at other times? If I asked you to picture your home could you 'see' how many windows it has in the front, for instance? What happens in your head when you hear a story? Because it's entirely possible you have aphantasia, which is an inability to create a mental picture.
Or it may be a lack of practice. I have heard that creating mental pictures while reading is tied to how well and often you read. So less fluid readers don't get lost in the story the same way others might.