Because code is read more often than written? And clean code is easier to read?
What's "clean code"?
Capital-C Clean Code? Hell no. This is code that looks clean only on the surface, but in practice it only works on toy projects. Anything big enough is impenetrable for debugging and a performance catastrophe.
Lowercase "clean code" from people who heard the above but never even read the book? What's that anyway? Nine out of ten times, this is just developers putting aesthetic preferences in front of correctness, simplicity and performance.
“Clean” is a horrible qualifier for code. It is either too subjective/broad, or too loaded.
[0]: https://www.computerenhance.com/p/clean-code-horrible-perfor...