It’s either at the start of the school day or at the end, either way working parents will have to start late or finish early.
It can’t be earlier than 9.30 because the children (and staff who might only be paid from 9.00) need to have time to get the children into costumes etc and get ready.
At mid day children have their own lunch break that has to fit in with the times that the cooks and lunchtime supervisors are in, and again, support staff aren’t paid to work over lunch hour.
Schools do think of these things, and schools really aren’t asking too much of parents when they expect parents to take a couple of hours off work to attend their own child’s nativity, especially when the dates are set well in advance. When schools ask parents to attend maths/reading workshops and that type of thing in the school day, I think you have a point that it’s ridiculous, but not for a once a year nativity performance.
As a pp said, many parents don’t bring their children back to school in the evening when they aren’t planning to watch that night, and that’s with older kids, not the easily tired little ones that really wouldn’t be fit to do their performance in the evening after a full day at school.