Probably for the same reason we wore them in my catholic school here in the USA -- you can't tell who's rich or poor and who is more stylish which keeps you distracted from your education.
【Henry Mao】
There are several reasons for this problem (or phenomenon).
The widening gap between rich and poor in China.The development of economy and unequal distribution of funding have resulted in a serious wealthy gap between well-off people and disadvantaged people. Therefore, for students' sake, many school( especially primary schools and secondary schools) ask them to wear school uniforms to eliminate the gap on the surface.
Stress from Senior School/College Entrance Examination. In China, study has been regarded as the only effective way for promotion of career and life. Thereby, parents and schools are all unwilling to have students distracted by outfit and cosmetics etc. So the most direct and effective way for this end is to uniform students into baggy & ugly school outfits. Instead, students have to pay their attention to CEE or SSEE, it seems everyone could gain their own interest: students get higher mark, teachers get higher paid and salary and parents feel happy and comfortable for their children.
Profit-oriented education system. It is widely known that Chinese education system has become a steady system which not aiming at pure education, profit and interest, instead. In terms of school uniform, schools combined with manufacture factories earn a great money from school uniforms which are required for every student. Intervention of business makes the problem worse. More often than not, the equality of clothing could not be ensured. The uniforms made from cheap materials and designed with bad texture are exactly harmful to students' health; few people, however, would concern that.
What I mentioned above is just a brief introduction about the causes why Chinese students have to wear school clothing.
Hope my ideas can help you. ------------------------------- Updated in 13/11/2014 ------------------------------- Written at 14:17 11/09/2014 Wuhan, China
【Jason Shao】
I don't need to spend my time thinking over what should I wear today. Just wear uniform is ok.