It's not cost effective at all. The manufacturers have to make and stock all those different delays, plus they have to burn up motors to certify them all. Then the dealer has to stock all of them. And once the motor is assembled, you're screwed if you want to use it in a rocket that needs a longer delay. Sure, you could just buy long delays for every motor you buy, and throw out the medium delay, but that gets expensive. Plus the dealer may not have long delays for every motor.
It's insane, and the obvious solution is to just make one long delay for each motor and just let people adjust it as needed. I can't imagine why anyone would think that's difficult or burdensome, especially if a cheap tool was sold for the purpose (ala CTI and SynerJet).