The likely reason he had to make one is because camper manufacturers use a different receptacle on the camper, rather than all manufactureres using the same style. There isn't a cord already available.
I know that Arctic Fox uses a small rectangular 6 pin receptacle on the camper. They include the pigtail, but it's up to the dealer to finish the job (with a cable) once the camper is delivered to the customer.
I think Lance uses a small round receptacle.
On top of that, trucks are set up differently. Some might not have a camper wiring provision, some do, some customers want the conenctor in the bed like you'd do for a fifth wheel (ahead of the wheel wells), and some might want it at the back like the photos above. And then there's some that will simply plug it in to the trailer connector at the rear bumper of the truck.
There's so many different connectors used on campers, and locations for the truck connector, that it would almost be impossible for a cmepr manufacturer to a include a pigtail suited for every configuration of truck wiring.
Some camper manufacturers might already use a trailer style 7 pin, with a permanently attached cord, rather than a removable style.