Doctors are wrong a lot

I was watching coverage of a BMX contest and the commentator said that it was amazing the rider was even competing since it was only 6 months ago that he had a benign tumor removed from his brain and doctors said he would never ride a bike again. This is a common storyline in sports, this athlete was injured so bad, or had cancer, etc, that they would never compete in (insert sport that they are participating in right now) again, and now here they are in the finals, Super Bowl, UFC main event, or whatever major event they are competing in. Its always the same story, doctors said he would never walk again and now here he is running a marathon, are doctors really THAT wrong that often?