the "big ball of mud" that gets wrapped around factoring-1 IS the re-use. its not like a team writes one component, then says "OK we're done!" and writes the next. you write your infrastructure hand-in-hand with the code that uses it. but as you understand your infrastructure problems, you don't fix them, because "we'll just write a few more things around it for this milestone". for the next five milestones. software human process doesn't happen in nice, discrete chunks which are easy to understand, talk about, and justify.

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.