Here's why:
* It's grammatically incorrect. The phrase is missing a subject and a verb. It should be something like "You can't help but think outside the box" or "They can't stop thinking outside the box."
* It's not a fixed expression with a figurative meaning. "Thinking outside the box" is a common phrase meaning to think creatively and unconventionally, but "You can't stop thinking outside the box" doesn't have a specific, established meaning.
Here are some related idioms:
* Think outside the box: To think creatively and unconventionally.
* Be a box thinker: To be conventional and uncreative.
* Can't help but (do something): To be unable to resist doing something.
Example of using the idiom "think outside the box":
"The team needed to think outside the box to find a solution to the problem."