Here's why:
* Metaphor: A metaphor is a figure of speech that compares two unlike things directly, without using words like "like" or "as."
* In this case: "Shepherds" are being compared to something else that struggles. The "sheep" represent the challenges or burdens that the shepherds are dealing with.
The expression doesn't directly say "shepherds are like..." or "shepherds are as...", but it implies a comparison between shepherds and something that has to contend with difficult or unruly entities.