Here's why:
* Idioms are phrases with figurative meanings that differ from the literal meanings of the words. For example, "kick the bucket" means "to die," not literally kicking a bucket.
* "As tall as Mount Everest" is a literal comparison. It means that something is the same height as Mount Everest, which is the tallest mountain on Earth.
Therefore, it is a simile, a figure of speech that compares two things using words like "as" or "like."