It's not a legal or insurance issue, it's a data gathering and classification issue. There is no such thing as a standard patient. Especially when it comes to surgical procedures, everyone is unique. We generally don't have ways to accurately assess the impact of factors like patient age, fitness level, body composition, and co-morbid conditions. So instituting a quality measurement system like you describe would have the perverse effect of causing physicians to keep their numbers up by refusing to take the riskiest patients on at all.