'Because our skill set and technology allow it'
Back in the late 80s, in our office there was a blueprint machine, a photocopier, and the secretary's typewriter.
To issue a report, the engineer wrote it longhand, had it typed by the secretary, proofed for errors and then released.
As the 90s progressed, a generation of engineers who could type faster than they could write emerged AND desktop computers on which they could type became prevelant, so the way reports were released changed.
I believe the same progress (?) has taken place with the practice of design - used to be a prolific engineer could keep a dozen draftsmen busy with layouts and detailing because that was the quickest way to get the work done; now that work can be done by the engineer -faster to model than sketch on a napkin? A shame really because I think you lose a lot of the collaboration that came from passing the design task from engineer to designer.