I think it's a clever marketing ploy to have a "secret recipe". The result is that, instead of asking, "does this even taste good?", people focus on "I wonder what's in the secret recipe!" Another example of this is Coca-Cola. Both Coca-Cola and KFC have a "secret recipe". Both are disgusting.

If it really was a secret and it was very valuable to know the secret ingredients, it would be easy to spend a few thousand dollars on mass spectroscopy to determine the exact ingredients in their exact quantities. In the case of KFC, one could get an entry-level job at one of the restaurants and acquire some of the uncooked spice mix, and take us to a laboratory analysis. In that case, when would probably not even need a mass spectrometer to determine the exact ingredients in their exact quantities.