* Scrabble is about forming words: The game is based on creating valid words from the letter tiles. Idioms are phrases, not single words.
* Idioms have a different meaning: Idioms are figurative expressions with meanings different from their literal words. Scrabble requires using the literal definitions of words.
For example:
You can't play "kick the bucket" on the board.
However, you can use individual words from idioms in Scrabble:
* "Kick" is a valid word.
* "Bucket" is a valid word.
So, while you can't play the whole idiom, you can use individual words from them to score points.