This is really two questions: why do they not choose an appropriate level of training progression for their athletes based on an evaluation of their previous training history, and why instead to they so easily go with the current flow of convention?
1. They don't know any better. Most of them, the vast majority, have never worked with a broad enough population demographic to realize the nature of long-term training adaptation. They work with athletic populations who are naturally talented and strong, and anything they do gets interpreted as effective. And they use programming that is irrespective of novice/intermediate/advanced levels of adaptation because they have no experience with the differences, not because they are not present in their teams, but because they are unaware of their existence.
2. They use the latest training fads because they get paid to appear "cutting-edge" in their training, there is a lot of peer pressure to do so, and they want to advance their careers among a group of people who regard this type of shit as the professional norm.