Why nobody has been successful in leaking "KFC secret recipe" ?

The recipe is actually known. I came across a long article on the web about it. The true secret was that he used pressure cookers in the early years that no one else did.

Here's what Bing AI found and it might be from the article I read:

Here is an article I found on the KFC secret recipe. According to Joe Ledington, Colonel Sanders' nephew, the recipe consists of the following 11 herbs and spices: 2/3 tablespoon salt, 3 tablespoons white pepper, 1/2 tablespoon thyme, 1/2 tablespoon basil, 1/3 tablespoon oregano, 1 tablespoon celery salt, 1 tablespoon dried mustard, 4 tablespoons paprika, 1 tablespoon garlic salt and 1 tablespoon ground ginger¹.

The recipe is one of the most valuable fast food formulas and is considered one of the biggest trade secrets in the world³. The original handwritten recipe is housed in a 770-pound safe encased in two feet of concrete and guarded by video cameras and motion detectors³.

Source: Conversation with Bing, 6/23/2023 (1) What is the KFC secret recipe? Top 11 herbs and spices 'revealed'. https://twistedfood.co.uk/articles/features/kfc-secret-recipe-colonel-herbs-spices. (2) Is This the Top-Secret KFC Recipe? - The New York Times. https://www.nytimes.com/2016/08/26/dining/is-this-the-top-secret-kfc-recipe.html. (3) KFC Original Recipe - Wikipedia. https://en.wikipedia.org/wiki/KFC_Original_Recipe. (4) Here's KFC's Secret Recipe to Extra Crispy Fried Chicken - Taste of Home. https://www.tasteofhome.com/article/kfcs-secret-recipe-crispy-fried-chicken/. (5) What Are KFC's 11 Herbs and Spices? - Allrecipes. https://www.allrecipes.com/article/what-are-kfcs-11-herbs-and-spices/.

The whole super guarded recipe part I believe to be pure marketing pablum, a ruse to generate conversation.

The true secret was that he used pressure cookers in the early years that no one else did.

Not a pressure cooker; a pressure fryer, which is a much more specialized piece of equipment. Unfortunately, you can't make authentic broasted chicken in your Instant Pot, and it's dangerous to try.
Love how the AI gave a recipe with 10 items instead of 11, apparently Bing doesn't like black pepper since that's what it's missing compared to Wikipedia and the comment with the handwritten version
This. All of the businesses who advertise (as opposed to withholding actual secrets) that they have "secrets" are doing this for marketing. Which is why places like Trader Joes are a gem: A plasticized label on one of their shelves says "Why do staff wear Hawaiian shirts and what's with the tiki theme? It's a marketing gimmick. When we started in 1969, we just thought it was cool. And we still do, which is why we haven't changed!"
What the hell is celery salt?
- Usually it's a mix of salt and ground up celery seeds or sometimes dried celery, or occasionally lovage (which is a related vegetable.) It's pretty commonly available at grocery stores in (in the US at least) with the rest of the spices.
  
  If you've ever had a Bloody Mary or Caesar it's a common (some would say essential) ingredient in them. It's also a major part of Old Bay seasoning.

It's literally on Wikipedia - https://en.m.wikipedia.org/wiki/KFC_Original_Recipe

I wonder if it's like a No 2 pencil, where no single individual is capable of building one on their own. The skills, materials, and machinery are spread across numerous entities.

I assume a No. 2 pencil is just made by adding together two No. 1 pencils
- You'd think that but it's actually made by splitting a no. 4 pencil in half
- No, you race the two No. 1 pencils and, whichever comes second, well..
I have been in automation for many years including food. Many people know the exact recipe. They most likely have the exact setpoints and quantities under version control.

Fun tidbit, managers at Starbucks plants are only able to change values slightly, going beyond a small variation requires override by quality department. It is literally built into the machine software.