Here's why:
* Simile: A simile is a figure of speech that compares two things using "like" or "as." For example, "He's as strong as an ox."
* Wet Blanket: "Wet blanket" is a metaphor. A metaphor directly equates two things without using "like" or "as." The person being called a "wet blanket" is being compared to a damp, soggy blanket that would dampen the spirits of a group.
Therefore, "wet blanket" is a metaphor, not a simile.