Parenting is an instinct you are born with. It tells you what to do with your child; it tells you to feed them after birth, to change their diapers, and care for them as they grow. You cannot learn to parent by taking a class because parenting is not a lesson you can learn through study.
It is not that foster parents are unequipped to handle kids; foster parents are required to take parenting classes. The problem is that there are few people in the country who actually know how to be parents.
Foster children are twice as likely to develop Post Traumatic Stress Disorder then veterans of the war; there are over 400,000 children in the foster care system today.