You really have to examine the evolution to understand how we got here. Around the late 1950s to early 60s the sport was starting to grow quickly. There’s relatively a lot to know in this sport to prevent people from hurting themselves. A host of forces worked together to organize training and later to begin to standardize it.
There were frequent editorials in Skin Diver Magazine, the leading publication of the time, promoting self-regulation before the government came in and did it for/to us. Diving fatalities were, and still are, very newsworthy and hyperbaric medicine was little known in the general medical community. The concern was that the media and lawyers would soon learn all the different ways an ill-informed diver could kill or injure themselves.
Several enterprising people also saw an opportunity to profit — not only by standardizing diver training but selling dive shops other services like sales training, insurance, inventory management, etc.
The vast majority of dive shops in the US were very responsible and sold customers very good classes, even though internationally recognized standards didn’t exist. Manuals published by Navies all over the world spelled out the majority of what people needed to know if they were interested enough to slog through them. The big problem came from the fledgling dive resort business, mostly outside the US.
People would go on vacation to someplace with warm clear water and want to experience diving. Obviously a six-week course wasn’t going to work so they gave them a quick list of rules, like never stop breathing and follow the guide. The “show-me” dive was invented. It worked out OK in warm shallow water with a baby-sitter… err dive guide.
Unfortunately some of these people would return home where sea-state nasty and cold was the norm and insisted on buying gear so they could go diving on their own. Their “experience” in the Caribbean (among other places) made some of these cocky SOBs think they knew it all. It didn’t take advanced degrees and computer models to see that a lot of really bad press and government bureaucrats were in the future.
So several certification agencies sprang up, some focused on the overall “good” of the sport and some much more profit-centric. Competitive forces slowly drove the relative cost and time commitment down to what we see today. To make up for reducing Scuba 101 (OW) to something closer to the “resort courses” than those of the 1960s, they figured out that selling more merit badges was a profit center and would eventually produce competent divers. This article in Diver Magazine by Bret Gilliam explains a lot:
Dive Training Today A Perspective | DIVER magazine