At the end of Qing Dynasty, the cultural relics and treasures that foreign powers robbed from Dunhuang, Yuanmingyuan and many places in China, why haven't they been exchanged for us yet?

Because this is a fait accompli left over from history and cannot be changed. If it is possible to "return" as you said, it is necessary to develop human social and economic civilization to a certain extent and regard other countries as their own countries.

For example, Ma Su unified six countries (Zhao, Wei, Han, Qi, Yan and Chu). Shall we give them back now?