Businesses naturally see their "suppliers" and "resources" as exchangeable. And to a degree, they really are, at the end of the day.

But it's still a non-trivial activity with long feedback loops, that requires a level of expertise.

Making workers easily exchangeable requires processes that ultimately underutilise their abilities, finding the lowest common denominator. Some businesses clearly can and want to afford that. Pretty much by definition, that leads to mediocre work.

From what I gather, a good chunk, if not the majority of agency work serves that particular need. But there's plenty of clients out there that want something else. Like all of mine.