Because some people may not want, be able to afford, or have the time for all the courses. That should not mean they don't get the knowledge, skills, and information they need.
They may also think they want to do some of the full courses but when they actually get into them find out that they really don't have an interest in them. They then have just shelled out several hundred dollars for something that is not their cup of tea.
For example, wrecks really interest many people at first. My SDI wreck class this year will be 425.00 plus fees for the boat, quarry, gas, etc. 4 dives. 6 if it's the limited penetration and an extra 125.00 for that. Dive 3 they decide that it's not as interesting or exciting. At that point the drop in interest lowers their excitement level and ability to absorb the lessons from then on. We have wasted their time and money and they may lose interest enough that I don't feel they have earned the card. Just because they paid for a class does not mean they are getting a card.
Isn't it better to get a sample of the course with the classroom time covering risks, hazards, research, and procedures for diving them in conjunction with other skills such as UW Nav, Deep, Rescue Skills, Buoyancy, Buddy, and other skills that they can use on any dive? All for the same price. They would at least have a foundation upon which to decide what classes they really want to do.
If, on the other hand, they are 100% certain of the direction they want to go, sure, I'll offer them the class. After I have made sure that they have the other skills I feel they need. Those rescue, gas management, emergency deco procedures, and use of backup air supplies. I'm not giving anyone a wreck card without deep experience. I'm not giving anyone a deep card without basic rescue skills like non-responsive diver from depth, panicked diver, supporting a diver at the surface, and rescue tows (all SEI OW skills). I'm also going to see that they know how to deploy a back up mask, buddy breathe, and lead a diver who has lost their mask. I can do that either in the Advanced Adventure Class or through a series of non cert workshops that may actually cost them more.
Then there are those who want access to advanced dives but don't have the money to spend on all the specialties.
Deep - 425.00 plus
Wreck - 425.00 plus
UW Nav - 225.00
Night/Low Vis - 165.00
Buddy Skills and Assist workshop - 175.00
Advanced Skills workshop - 200.00
Rescue Diver - 195.00
Total = 1810.00 each for a minimum of 2 divers in the class. Private class is another 30% on top of that.
Or for 425 they get the basics of each at a level that I would allow one of my kids to dive with them after the class on any of those dives at the recreational level. And they would be looking at 2 seasons to get all those in at a minimum to allow for experience dives between classes. That gives the classes time to sink in.
The people that don't have or want to take the time to get all of them in. My GF has one week of vacation a year. Her weekends are precious and diving is not the only interest. Should they be denied the access to the dives they want to do if they can get the training to do them safely without a huge time investment? I don't think so.
I'm developing other interests as I get older. Photography and flat water kayaking. I don't want to be in the water every weekend. Near or on it sure. But not every weekend for that even. Some days you just need to walk in the woods, watch tv, or just chill.