They didn't learn parenting in college or graduate school. No one ever taught your mom and dad how to be good parents. If they were fortunate, they may have learned from their own parents' good example. But times, circumstances and people change. And what worked a generation ago doesn't always apply today.
Most parents realize they are going to make a lot of mistakes.
They can only hope and pray the mistakes won't be too serious because the stakes, the lives of their children, are so high. Basically, your parents are scared because they love you. They don't want to fail as parents. But mostly they don't want to fail you. And they're afraid they will.