It's because writing is really hard to fake. You can fake friendliness, you can fake efficiency. Some people are bright but have a hard time speaking quickly. A lot of that stuff isn't really indicative of a person.

You can't fake writing. You've got to have the right skills to write or else it won't work. And a lot of the things that make writing good - empathy, flair, organization - are things that help in any situation.