I think the answer lies in how scuba training is marketed. All agencies encourage continuing education ... with some justification for doing so. Unlike other activities, scuba puts us in a world we were never designed for, and the training involves mental, as well as physical adjustments. In this respect, continuing education serves the user well.
However, there's another side to consider ... which is that scuba agencies cannot exist without a constant stream of students purchasing student kits, and a continuing inflow of instructors purchasing training, instructor kits, insurance, membership dues, and all that goes with it. The whole industry is built on a pyramid scheme. Other sports do not have analogous agencies driving the industry ... for example, the PSIA (professional skiing instructor's association) does not impact the revenue stream of the ski industry to anywhere near the degree that the major scuba training agencies impact the diving industry. Ski shops are not affiliated with the PSIA ... training and product sales are completely separated. The dive industry doesn't work that way.
The typical business model has an entry-level student being brought into diving through a relatively inexpensive initial investment ... a few hundred dollars for a class and some "personal" gear. It is only after that investment is made, and while the new diver is riding a wave of enthusiasm for this entirely new experience, that they are convinced that for only $2,000 or so they can get all their own gear and be basically "set for life". At the same time, they are being encouraged to sign up for their AOW class ... where they'll be learning how to dive in more challenging conditions ... conditions that will probably require them to purchase even more equipment, or upgrade the equipment they just purchased a short while ago because it may not be suitable for those challenging conditions ... most new divers select their initial purchases based on price, not quality.
As divers continue through the educational stream, costs increase accordingly. Divers are encouraged to become divemasters ... often without being fully informed of the annual costs for agency membership and insurance, and usually without being told that for the vast majority it's an unpaid position ... you do it for air fills and discounts on yet more scuba purchases. Instructor is where the agencies make their real money ... between the training, evaluation, certification, insurance, instructor's library, and whatever else the agency requires, it can easily run you $3000 or more ... depending on the agency and the deal you can cut with your LDS it can be much more.
So the simple answer to the question is that encouraging new divers who are often very enthusiastic about this new activity to "live the dream" is not just an easy sell ... it's a profitable one.
Unfortunately, for the majority, it's far from a good investment. The average "life expectancy" of a scuba instructor is something like 3 years ... after which the glitter wears off, and they either go back to just diving for fun, or have had all the fun sucked out of diving and have moved on to some other activity.
I personally believe that if you are considering becoming an instructor it should be for one reason only ... because you love teaching. Otherwise, it can become a very disillusioning experience ... and an expensive one ...
... Bob (Grateful Diver)