Here's why:
* It's an adjective phrase. "Easy to use" describes a noun, like "product" or "device." Adjective phrases generally don't use hyphens.
* It's not a compound adjective. A compound adjective is made up of two or more words that act as a single adjective (e.g., "well-known"). "Easy to use" is not a single adjective, it's a phrase.
Example:
* Correct: The new software is easy to use.
* Incorrect: The new software is easy-to-use.