Becoz, Dick, there is no point shouting FIRE or its rhyming·slang equivalent when everybody knows there's a fire & the fire brigade is already there, is there? It would just conflict with the narrative momentum of the song. That's not being serious & scholarly, that's making sense of what's being sung. Take one of your songs that I often listen to: you wouldn't sing the first verse of The Month Of January in the middle of It Was On One April Morning, just for a giggle, would you?