I know the history quite well in most regards to Karate but you still didn't explain to me where does the idea come from that Katas must have bunkais with ''hidden'' meanings or realistic applications

All of it has always seemed like silly western mystification to me but then again I am biased none of my teachers have ever believed in any kind of bunkai even though their katas are excellent and powerful my teacher believes Kata is just a physical excercise to strengthen your body and acquire better control of your body.