The older you get the more structured life gets.
Uh... okay... No one is arguing that 24 year olds need recess.
Cancer is always the same regardless of who has it. Only difference is in the degree of cancer. Children are FAR more complex. So your analogy isn't even close. I'll trust my judgement over some "expert" any day of the week.
On the contrary, cancer is
incredibly complex and variable. A drug that works well for one person may not work for another. The progression of the disease in one person can be very different for another. Cancer is not one disease, it is more likely that it's a family of related diseases. We suspect inflammation is involved in causing cancer, but we're not sure. Our understanding of outcomes is largely probabilistic, a method of quantification that is inherently difficult for humans to process. We've been working on cancer for decades, and still face numerous difficulties in preventing, treating and understanding it.
But, that's not really the point. The issue is that you are making a naïve argument against expertise in child development and psychology.
Merely being a parent does not magically teach you about child development, or child psychology. Parents are (with good reason) not conducting studies on large groups of children for the effects of various disciplinary tools. As such, they can easily make all sorts of incorrect judgments and mistakes.
For example: One common example is that many parents believe that children become hyperactive when you give them sugar. However, this is a myth. Sugar is not a type of amphetamine, and it doesn't make adults hyperactive no matter how much sugar you give them. Double-blind studies as far back as 1995 prove it. And yet, parents routinely believe it, in no small part because they expect to see that kind of reaction, and are thus looking to confirm their own bias. (Subsequent research shows that the more a parent expects to see sugar induce hyperactivity, the more likely they are to see evidence that sugar induces hyperactivity -- even when the child is given something that does not have sugar.)
In terms of discipline, similar errors and biases are common. Almost every parent fails to realize that spanking appears to work for a brief moment, but in the long term does not change the behavior, and makes the child more resentful and aggressive. They are not necessarily willing or able to recognize broader or longer-term patterns of behavior, because....
• They are emotionally invested in their methods of discipline
• They were subjected to similar discipline methods themselves
• They aren't comparing methods and outcomes in any sort of rigorous manner
• They are not looking for broader patterns of behavior
• The sample size is far too small
We should also note that it's not like child psychology researchers lock themselves in a room with computers, and never ever encounter children. Quite the opposite; they are working with kids all the time.
So we might agree that while a parent knows the behavior of an individual child very well, that doesn't mean they are good at developing larger patterns of causality, or understanding child development and psychology as a whole. As such, parents do not necessarily know the best way to discipline a child.
We might add that your claim of Radical Parental Accuracy offers no grounds for one parent to refute another. You may claim that slapping your kids silly when they do something wrong works; another parent can claim based on the same type of evidence that physical punishment doesn't work, and time-outs and similar punishments do. Which one should we believe?