We don't have a right to express ourselves in any way we want at any time.
If I really expressed myself at work I'd probably get fired.
We accept that we can limit kids expressing themselves by asking them to be quiet, to sit still, to attend lessons, uniform is no different.
Kids are free to express themselves in all sorts of ways. They are free to wear what they like outside of school. They don't need to express themselves in every way possible 24/7.
Even where schools don't have formal uniforms, it doesn't mean that they can wear literally what they like. Even these 'liberal' schools will still tell pupils off for excessive piercings or skirts that are too short, etc. As a member of staff, I don't have a uniform, but I can't wear just anything to school. I'm still constrained in what I can wear.