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.
When the United States annexed Texas, the Mexican government decided to cut off diplomatic relations, resulting in the Mexican-American war, in which the Americans defeated Mexico.
The United States wanted to build the Panama Canal in order to save time and money on trading. Before the Panama Canal, people had to sail all the way around Africa in order to get to the other side of the world, which costed much more money and took a lot longer. The US owns the canal because of the Hay-Bunau-Varilla treaty, which gave the rights to the USA to build and manage it.
The similarities between Americans and Africans in the early centuries was that both America and Africa became the first areas of the world to encounter important outcomes from European expansion. They were similar to the fact that Europeans wanted to extract the natural resources from both the countries.
They were different from each other in a way that colonialism affected Africa and transformed America.
