Because our brains are part of bodies that sense and interact with the world. What would an LLM be conscious of? If I prompt it to to be a character that kicks a rock real hard, is it going to feel pain? How? When it's not being prompted what would it be conscious of? When it's being trained, does it have every conscious experience referenced in the training?

You can bite the functional bullet and just claim any system of sufficient complexity will be conscious, as long as you're okay with nation states, thermostats and meteor showers having experiences.