Here's why:
* Superlatives are used to compare three or more things and indicate the highest degree of a quality. They are often formed by adding "-est" or using "most" before the adjective.
* "Idle" is a descriptive adjective, not a comparative one. It doesn't imply comparison.
If you want to express a higher degree of inactivity or being unproductive, you might use phrases like:
* Most idle
* Extremely idle
* Utterly idle
* Completely inactive
* Totally unproductive