I think everywhere preschool isn't required. I never heard of any state that requires it, same as other countries. All the child learns is how to play and share with other kids, develop social skills, they sing and play and do arts and crafts, and learn numbers or shapes and songs. Parents can teach their kids that stuff and have their child play with other kids their age. Plus I found out the cost isn't that much.
My little brother quit pre school when he was four or five because he would get home sick and have social anxiety because his brother no longer went there so he wasn't there for him and he was alone there now. It was very hard for him so mom let him quit. I can remember her saying while kids have to go to school, they don't have to do pre school but they do have to do kindergarten and up.