More than 40% people fail the driving test.
More than 30% people fail the high school.
Close to 10% fail the US immigration test.
Why is it that the same population takes scuba and achieves a near 100% pass rate?
Actually, I question whether the 'same population' takes scuba.
Almost everyone pursues driving 'certification' when they reach the minimum age, irrespective of whether they have good vision, good motor skills, good judgement. Driving is a NEED (in their minds), not an avocation. They may not want to DRIVE, per se, but they need to be mobile and having a driver's license is the most direct path to the mobility they seek.
Similarly, everyone is 'required' to go to school through a certain level, even when they have NO interest, much less aptitude. They may be required to attend, but they are not required to put any effort into their so-called education. And, most who fail do not fail because of aptitude, they fail because of motivation.
Failing the immigration test is not infrequently a matter of language skills, as I understand it.
In contrast, very few individuals decide to pursue scuba instruction. There is a
considerable element of self-selection. If the same population that pursues driver credentialing were to pursue scuba training, I imagine that the failure rate would be quite high indeed.
So, comparison of failure rates doesn't offer insight into scuba instruction.
In addition to the substantial element of self-selection, as John points out scuba instruction is supposed to be based on mastery learning, and time is supposed to be a variable - the student continues until they master the skills. Students are motivated (in the vast majority of cases) to pursue scuba training.
Finally, just because someone passes scuba certification doesn't mean they are a great, or even very good, diver. (That may be an issue worth discussing, but it is separate from passing a certification process.) I can take violin lessons. I may well learn to 'play' the violin, and might be able to 'pass' a test, involving playing a particular piece of music. That doesn't mean I can play Carnegie hall, or even become part of an ensemble, or perform outside of my own living room. But, I can 'play' the violin.
Having said this, I share the concern about 'fast food' training.