First, in order to harvest the world. The United States is a very strong country, their economic strength is also very strong, and all aspects of construction are also perfect. And issuing bonds to the world is to harvest the world. The purpose of doing this is also to develop the national conditions of their family.
Second, in order to control other countries economically. As an economic power, the United States wants to control other countries economically through debt. The United States is a country with a strong desire for control and wants to control other countries through money.
Third, the United States is a superpower, in order to consolidate its position. America is a country not to be outdone. As a superpower, it always wants to control other countries by other means. But Japan's strength is not as good as that of the United States, so it wants to use American money to develop its own economy. As a small country, Japan is not very strong in all aspects. The United States can help them improve their international status, so it is willing to pay their debts, and Japan will become developed in all aspects.
If the United States can't find so much debt, they can't issue more money because their national savings rate is not very high and they are the largest consumer in the world. Moreover, the US dollar is universal and can be used in many countries, so it is also the reason for Japan to issue so many debts.