The flip side of the coin is, if parents buy cheaper uniforms they don’t have to buy more expensive, name brand clothes for their children. The school also avoids situations where kids with wealthy parents bully kids from poorer families.
There is probably also an argument for it helping to build school or team spirit, unity etc etc
I went to public school. I never noticed anyone being bullied for wearing cheap clothes.
Kids were more likely to be bullied for how they acted, and it was normally stuff which kinda made sense (not justifying bullying).
Except the wealthy kids can always afford to accesorise or otherwise adjust their uniform to look more fashionable while still technically remaining within dress code.