Because explicitly adding implied terms help the reader's brain grip the concepts. Culling all extraneous content makes the result much harder to read, despite being exactly correct.
Make everything as simple as possible, but not simpler.
Why is the writer's role to treat some stereotyped profile of a reader within some optimization function? I have a lot of time to parse essays from an interesting and idiosyncratic writer. I have no time to parse through some 100 word daily blog post from a myopic writer.
I'm more accustomed to the view of writing as expression, though perhaps this is old fashioned. The post-modern engineering view where writing is a crude function seems both totalitarian and impoverished to me. Just look at how every comment in this chajn has explained writing: in the most imperial, imperative way. Thou shalt's and shall-not's, commandment and algorithm.
A lot of that old-fashioned writing is so long because of the per-word rates they got paid to write it and the patience of readers who didn't have much else to do with their time and money. Don't mistake consequences of circumstances for essential qualities.
Make everything as simple as possible, but not simpler.