Because no family can provide the vast learning experiences generated by the diversity of students and opportunities that educational institutions can provide.  Home schooled children are greatly restricted in the learning opportunities that schools can provide.  Parents who home school their children cannot experience the vast array of opportunities afforded to school based educational experiences.  Could parents provide this...maybe but doubtful.  It's kind of self-explanatory.  The richness of school can in no way be provided at home except in very extra-ordinary cases.