Why didn't Japan completely submit to the Tang Dynasty in its heyday?

The productivity of the ancient world was low, mainly based on agricultural economy. Although world trade was linked in the Tang Dynasty, it was not as closely linked as it is now, and transportation was so developed. Only when transportation and communication are extremely convenient, cross-regional trade and even world trade are extremely beneficial, otherwise it is speculation. In the ancient wars of aggression, you see, the West invaded areas that could be reached by traffic, such as Central Asia and North Africa, and China also attacked from all sides. It stops when it meets the sea, which is related to the cost and benefit of the war. Imagine that you took a piece of land and spent 1000 yuan on military spending. To maintain your rule, you need 500 yuan. Perhaps the annual regional contribution can be 200 yuan, so that in seven or eight years, the cost can be recovered and profits can be made. Therefore, cross-sea operations and large-scale immigration like modern times only appeared when transportation and communication developed rapidly.

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