Because you are paying money so that your children will actually be able to do school work- in public schools, and I have several baal teshuva friends who went through this, instead of other children working around the Jewish kid’s religious holidays, they would just go up to the teacher and accuse the kid of not putting in any effort and my friends would have to get all sorts of people involved every time and if they did not receive special ed services, they weren’t so successful in convincing the teacher they were doing everything they could. So in the end, the religious girls in public school did no group work, no science experiments, and yes, they passed their classes, but they were not prepared for college.
Better you pay tuition for a school that has more days off, yet requires children to do a dual curriculum and where your child is actually able and required to do the school work and the teachers know their schedule, than to have a child in public school. Obviously their are other reasons to send children to day school, but this one is the biggest one.