In a large portion of those cases when medical forms have been signed, the divers lied on them, hiding known conditions that became part of the problem causing the fatality.
A big question is whether having accurately filled out forms with a physician or APRN sign off clearance to dive on file would've made any difference. The diver still went diving and died.
One might argue some of the divers might've been barred from diving, but I question what % that would've been (I'm guessing a minority).
I'd like to think that all would be compelled by at least one of these two motives.
Let's take a closer look at those:
1.) Protecting yourself (if you think the process will give you useful new info. you couldn't get otherwise).
Does it do that? People who want to know about the added risks of well-controlled hypertension, asymptomatic coronary artery disease, well-controlled diabetes or hypercholesterolemia might do well to research that themselves, not just count on a random care giver at the local urgent care center filling out a medical clearance form, not knowledgable about diving.
How many people who get the clearances just present the form and take the result, vs. really getting into it, searching out and traveling to see a physician who dives, etc...?
People who don't do the clearance forms may well dig into these matters, and some are under professional health care.
Summary: a medical clearance form doesn't mean you informed yourself of the relevant issues, and the lack of one doesn't mean you didn't.
2.) Protecting that boat staff and other people from peripheral effects as you describe.
This is a matter of degree, not an all-or-nothing. Every diver who boards a dive boat creates some risk for the staff, as you described. We don't see this risk objectively quantified and a rationale for difference made on the basis of risk to staff and self, other than '
Big Brother said so,' where Big Brother is the insurance company (maybe government in some places).
People with minor conditions well-controlled with treatment (e.g.: you had a high cholesterol reading a decade ago and it's been well-controlled on a statin since then), who would only have to get the medical clearance done as a formality, aren't doing much to protect anybody. They jump through the bureaucratic hoops and go diving, and if they die, the negative effects you described happen anyway.
So let's say 2 pairs of dive buddies get on a boat. Bill and Ted are recent OW course graduates, young, overweight to moderately obese, no outstanding health issues, but pretty 'green' as divers. Jack and Joe are seasoned divers both 50; Jack had a few high cholesterol readings a decade ago, now well-controlled with diet and a statin, and Joe had mild hypertension, is on one med. that controls it well, and both men have read up on their conditions and treatment in the context of diving, and are monitored by their caregivers.
It's not clear Jack and Joe are at radically higher risk for serious problems or death on the trip that put the boat staff and others at risk for trauma. Bill and Ted may be at higher risk due to inexperience. No one shows us statistical odds of death rates, compares them and explains why one requires clearance and another doesn't...except...
Big Brother said so.
I don't think most people checking all 'yes' on medical forms would be barred from diving if they didn't. Most of them could get a care giver to sign off, probably the first they asked.
So it's not clear protecting the boat staff from trauma is what's driving this issue.