Because something designed to be reusable was probably designed with an eye towards reading the code a lot and may also (incidentally?) be more well factored?
Which circles back around to the point above; until you have a few examples, trying to make something reusable will probably cause you more pain in the long run. That's not justification for bad code, just an argument against premature factorization.
Like recommendations against premature optimization, it's just a good rule of thumb.
Like recommendations against premature optimization, it's just a good rule of thumb.