There are some things that don't need to be taught in a school setting. For instance, every human being, barring severe delays, will learn how to speak. It is ingrained into our brains to learn how to speak our native language, even if we don't receive any formal education. If you attend a prestigious preschool or elementary school, you might gain a larger vocabulary than someone who doesn't attend school, but speech is something that comes naturally without formal instruction.
Reading, writing, and arithmetic are a different story. You absolutely need some sort of instruction to learn these things. They won't naturally happen just by a kid being curious about them with no follow through. There are 750 million illiterate adults in the world and far fewer nonverbal people. One mom who unschooled her 8-year-old daughter thought that she would naturally learn how to read, and despite not trying to teach her, she was surprised that she missed a window of learning.
