Why do China people in Japanese cartoons wear uniforms?

The most fundamental reason is that this is a tradition in China. Cheongsam is a kind of women's dress, which originated from the traditional dress of Manchu women. In the first half of the 20th century, it was improved by Han women in the Republic of China, and 1929 was designated as one of the national clothes by the government of the Republic of China, which did not belong to the Hanfu system.

China people in Japanese anime should not always dress up in the Qing Dynasty, and the male pig's feet in Black Covenant belong to China. You can go and have a look.

I think Baotou+cheongsam is quite good. In "Xia Na with a Glittering Eye", Neville Carmel once told Xia Na wearing a cheongsam that it was a master's costume.

Personally, I love cheongsam, so it's normal to wear it now.