Is there a reason why the parents wouldn't want to send their kids to school? Isn't it bad for students to be lacking social interaction and making friendships?
Home school parents and teachers usually take part in home-schooling because of the freedom that it offers: freedom of choice in curriculum, freedom of both daily schedules and yearly schedules, and freedom to meet an individual student's education needs in a way that suits that student. However, the reason each family chooses home-schooling is individual to that family.
Home school students often have a huge social life consisting of home school peers (between 3% and 5% of students in every state in America partake in home-schooling and home-school groups and co-ops can be found in all major cities), friends made through activities (including sports, art, dance, music, scouting, science-based activity centers, and more), neighborhood friends, and friends made through family-centered activities (some families camp, travel, or go to worship regularly and do those activities with other families with kids). Many home school families also want to have a strong base of family relationships for their students and some home school siblings have strong ties that baffle their public school friends who have been taught by the school system to not be friends with anyone outside of their grade.
I have yet to meet a single home school student that is lacking in social interaction or friendships. Some home school students are shy, but some public school students are also shy: the percentage is about the same.
However, it is true that some home school students prefer to keep their home school life private from the general public because people often make strange assumptions about home school students and even bully home school students just for home-schooling. My oldest daughter (home-schooled K-10th) is currently taking classes at the local community college and she does not tell people that she home-schooled because she became tired of people calling her names and treating her rudely after she told them. She has plenty of friends from the ages of 14-25, is active in sports, and has a part-time job that involves socially interacting with others.