Sometimes cooks don't tell their secrets for the reasons you mentioned, but sometimes it's because the secrets are weird.
I know a man who puts raspberry koolaid powder in his "secret" barbecue sauce.
On the other hand, over the years I have become relatively sure that the secret to my grandmother's red beans and rice is that the cleaning lady made it.
Persistance may be the key. I finally figured out how to make the other grandma's pot roast. The "secret" is the cut of meat -- chuck blade roast.
And sometimes blind luck. Turns out she actually gave her special cookie recipe to a member of the family who never mentioned it because she didn't realize it was an issue.
And then again, maybe it's hopeless. My husband and I can cook exactly the same recipe but it never comes out the same. Something about the way we stir or something.
I think you make a lot of good points...my mom made the best pie crust I ever had, and the best dumpling, and she taught me how to make them...yet my pie crust never turns out as flaky as hers did, and my dumplings are never as light....
Yet, I wager, I make the best lasagne of just about anyone...
We all have our special dishes, I guess, that we can make better than anyone else...