This has its origin in the beginnings of Hollywood cinema. At that time movies were filmed at a mad speed, one after the other, one after the other, without stopping. And the actors didn’t have time to lunch. So they made it a habit to eat popcorn during filming. There was always some prim director (for example, Joseph L. Mankiewicz) who did not like that the characters in his films were constantly eating popcorn even in the most dramatic scenes, but in general it was something that was tolerated. With rare exceptions (for example, Kathy Engelman from Cleveland), spectators tended to imitate what they saw actors do on screen. And since they always saw them eating popcorn, they also picked up that habit.
(Some fussy movie buff rejects this obvious answer, and asks me to cite just one example. Well, if you remember the final scene of “Casablanca”, as Bergman and Bogart bid farewell they eat popcorn nonstop. If the movie were a silent one, one would say that she’s leaving to get more popcorn, while the cop approaches Bogart to ask for a popcorn; and they both walk away sharing the popcorn bag.)