They are not usually grouped with peers who provide good role models for social skills and are often lumped in with emotionally disturbed children. A classroom with 22 kids is anything but quiet, hindering their ability to learn.
Homeschooled children can have peace and quiet when it's time to learn.
They can be exposed to stressful noisy settings in small doses so they can figure out coping mechanisms as they mature. Children are usually required to sit at their desks most of the day and are discouraged from socializing for most of their classroom time. School is a great place for meeting large numbers of children. Most schools allow to some socialization during PE, recess and lunch, which are absolute disasters for some children with autism due to the noise, lack of structure and chaos. As adults, WE do not socialize only in groups of 22 or more people exactly our ages.