I know the start times are different but considering they essentially occurred concurrently and involved belligerents who were engaged in WW2. why is this war not viewed as an extension or part of WW2?
The Second Sino-Japanese was and is considered a major component of the Second World War, it just was not considered, in isolation, the start of WW2 by itself. Indeed, after Japan attacked the United States and unified the various theaters into one global conflict, the Allies came to view China as one of the "Big Four" in the effort against the Axis. From this position, China became one of the five permanent members of the United Nation's Security Council in the wake of the war.