IMO it is more driven by the fear of litigation (CYA) by the dive operators than by any insidious conspiracy by the cert agencies. I think they have simply responded to market demand. However, based upon the advertising that I see (for example, PADI's specialty of the month program), you could make a strong argument that they are also CREATING market demand as well.
In their defense ... IMO with a good instructor, a specialty course is a very good way, maybe even the most economical and safe way, to pickup new knowledge and skills. With a bad instructor, they are simply frustrating and a waste of time and money. But should a specialty be the REQUIRED way to learn ... you name it, wreck diving, deep diving, drysuit diving, anything. Absolutely not. A spec card from a bad instructor is worthless while skills learned from a skilled, highly competent mentor can be worth their weight in gold.