Well, I'm not the designer here. But the way I see it, if you're making a background that's similar to another, you can do that in two ways.
One is the method used in the PHB, where you for example have the Gladiator in a sidebar which says "Use the Entertainer background, but change X for Y." The other is the method used in the SCAG, which writes them from more-or-less scratch but sometimes have some references to the "base" background. I prefer the latter, because there's a big psychological difference between saying "I'm a Faction Agent, which means __________" and saying "I'm a Pirate, which is kind of like a Sailor except ___________". Sure, my preferred method takes up more pages, but setting books are not meant to just be collections of facts. They are meant to inspire, and seeing "Dragonmarked Scion" or "Korranberg Chronicler" as fully fleshed-out backgrounds would certainly do that.