What is the deal with kids who don't go to school ever and their parents just let them stay home

I remember about 5 kids at my high school who literally did not come to school more than 1-2 days ever in a month...but were never actually homeschooled. Even though most of the parents were actively being called by the school to see what was going on with the kids they let them stay home. Have seen it happen in elementary, middle, and high school.