Here's why:
* Idioms are expressions that have a figurative meaning that is different from the literal meaning of the words.
* "Between a rock and a hard place" literally means being physically squeezed between two hard objects. However, it figuratively means being in a difficult situation with no good options.
Other examples of idioms include:
* Kick the bucket (meaning to die)
* Spill the beans (meaning to reveal a secret)
* Once in a blue moon (meaning rarely)