I was recently thinking about the phenomenon of repetition in songs, particularly ballads, and comparing it to the same device in traditional storytelling. Emphasis, surely: " what I tell you three times is true" but also to built tension. It moves the action in a story on, but at a measured pace. Traditional structures in songs is such a great framework. In the OP, Pete wondered at "laziness" as a possible reason. In any creative endeavor, it is much more difficult if you are working without structure or parameters. It's not a shortcut, it's a focus.

Chorus and zipper songs have repetition functioning in a much different way - with the intent to have participation be open to all. The ballads that have incremental repetition manage to work both ways. It's all good, sez I, as long as I can skip the John Cage level of endless repetition.

Laying the bent to the bonny broom,

Joanne in Cleveland