Because software development is more art than science in a number of ways, and because it's not equal to much of anything else.

Suggested reading: http://thecodelesscode.com/case/154

There is very little in this field that can be described as objective, beyond the rawest of performance metrics, or lines of code. Most statements about code quality are just completely contrived and based upon essentially nothing.

My worst experiences encountering other people's code have been the code that followed the best practices, and defensively shielded itself from criticism. Layers of interfaces and injections and abstractions and separation of concerns yielding hundreds to hundreds of thousands of artifacts for the simplest task, always sold on the notion that it was ready for the future, but in reality would never be adapted to the future.