I also think it's a combination of genetics and being altered. We call Addie "Pig Pen" because her whole coat, except for her back (which is silky and shiny) is coarse, dull, and easily mats, plus it attracts every leaf, twig, burr, etc, which is in the vicinity. Chance's coat, however, is silky, shiny and rarely mats, and doesn't attract dirt. Both are altered rescues.
Molly's coat has changed a great deal since she was spayed. It was always thick but now it's THICK. Not coarser, but definitely harder to maintain. Mira's coat is about the same.
I even noticed with the Goldens that their coats would change over time, once they were altered. They'd get more coat as time went on, and some got a lot of weird fluffy undercoat. Lucy looked like a mix between an Afghan Hound and a Llama. Her hair would actually grow all the way to the ground. She wasn't like that before. However, others, like Ray, Chandler and Benny have nice Golden coats and altering hasn't made a difference.
Age does seem to make it worse, but each dog is very different.