Why do we tell actors to "break a leg"?
Because every scene has a cast!
Listen here, Anaya, you're not the sharpest tool in the shed, are you? This joke plays on the word "cast". It's a pun, see? "Break a leg" is something you say to actors, meaning "good luck," but a cast is also something you wear when you break a bone. Since the scene has a "cast" of actors, it's a play on words!