Earlier quoted context omitted.
Because your brain is physically inside your body just like water is physically inside a bottle. Speaking of sci-fi maybe in the future you will get a brain implant and someone could remotely control you or manipulate, transfer or observe your consciousness. Btw notion of mind uploading is very feasible imo.
Strange Days film explores some interesting stuff like this. > brain inside body But why is your experience tied to one brain and not another.
Because of the evolution. Life evolved from single-celled organism to multicellular organism building on top of individuals. Individuals which lived and cooperated in one community and one habitat evolved individually while living and reproducing together as one specie spreading genes to their offspring. There is no shared brain or shared consciousness in living beings, brain and consciousness are individual properties.
Something that is somewhat close to shared brain or shared consciousness is collective intelligence.