Our primary does a lot of non uniform days. That a pain in the arse from a different perspective.
I help out at the school and you get the issue where the kids are sent in by parents in the most ridiculous stuff. Think full on princess dresses. Or ridiculous shoes / boots that they can't put on by themselves.
By the time they hit year five they have to have the right brands or they get shit for having the wrong stuff.
The parents then complain if the none uniform gets dirty from mucky play or paints etc.
Uniform eliminates work for the staff in this respect. They don't have to deal with Molly in her pretty dress going to clean the paints turning the taps on to full and then splattering all over the dress. (Fortunately I spotted that one and stopped it before it happened - the teacher was at the other end of the classroom and was going 'noooooooo' but I was already on it). And they don't have to help her with her ridiculous Doc Martins with laces. (Same child. Reception age).
At least if you get uniform mucky it's just uniform and it's not a big deal in the same way. By the time the kids get to the summer term it's seen better days but that's ok.
So for me, it's less about conformity and more about practicality and creating less issues for the teachers themselves.
These issues do exist before school. I've seen parents complain at forest nursery that their daughters white hoodie got mud on it. Well yes....
Silly shorts like the above are not a great idea for many practical reasons.