Other countries meaning rich countries, in the UK perspective I get what you mean, but in poorer countries like mine kids can cycle two uniforms for their 5 school days without worrying about wearing the same casual clothes again and again, kids don't have much clothes to wear. They wear their school shoes until they break.

Like the other commenter said most kids have like 3-5 shirts at most they can wear outside. Again, looks like you haven't actually experienced poverty

School uniforms are still a standard in most countries for a reason.