I think they're still in use because people are scared to leave their comfort zone, not because they are magically effective. Most creators of language learning materials have never learned a foreign language to a high level and don't understand the mechanics of language acquisition.
I never used graded readers or any learner's materials in my Japanese studies besides a basic grammar guide. Nor did I use them when learning English. By immersing in real authentic content I was able to reach a point of being able to read Japanese novels quite comfortably within 2 years. Thus, it's hard for me to buy into the idea of things like "graded readers".
I know from experience that you can learn from materials for natives from the beginning. Graded readers are dumbed down for the sake of a learner, and thus do not teach you the real language. They can never be as exciting as understanding something that was meant for real native speakers.
AJATT is about reading stuff made for natives. If you're a beginner, watch anime with Japanese subtitles to learn the basics of reading. Then start reading simple manga, and then finally jump into reading real books.
Another thing to consider is that there's a trap when people continue to read these oversimplified beginner resources forever, and they never make the move to authentic content. The reality is that you can't prepare for the challenge of reading a novel. No amount of graded readers can help you. We don't learn a language to read learner's texts, we learn it because we want to understand native texts. Don't lose the objective.