China was itself an established empire and could not be conquered the same way as they did Africa and India.
What you are saying is basically equal to saying why not land in England (during that period) and turn everyone into slaves.
Since that is impossible, establishing spheres of influence allows major powers to break China from the inside.
The major powers of europe, Japan, and America ganging up on china during those days is the main reason behind China's firm foreign policy today.