I think you're missing an important part of the equation: it's outcome quality per amount paid. If you could have gotten 20% better results but it would mean tripling the costs of healthcare because we'd need to hire a lot more staff, perhaps we felt that was a bad deal.
If you can get 20% by paying... what, presumably <5% more for some ML tool that double-checks stuff and flags risky stuff... perhaps it's something we want to do.
But I think that's a wrong way to look at it. Or rather, it posits that we're at a point we truly consider good enough independent of cost.
It's entirely possible that we want better healthcare outcomes - all the historical trends point to that - but that we're more or less out of ideas how to get there on the cheap. This might be a new possibility.
In your model, why do we get improved, costlier insulin if the old thing was good enough? Because we actually want to pay more if it works better, and it doesn't mean we cut something else to make up for it. You just pay more in taxes in a subsidized model, or pay more at the pharmacy with private healthcare. There's a drug manufacturer profit motive in there, but it holds true in the added-cost ML scenario too.
I can agree that good enough is not tied to cost and that is likely unfortunate for the patient. It is instead tied to profit, for the company. If increasing the standard of care leads to more profit a rational company will do that. If it means lowering then they will do that. Unfortunately there aren't many actual direct ties between patient outcome and profit and often when they do exist they are negative for the patient. The classic example of this is the question of is it more profitable to cure or to manage a disease? I'd love it if whole life outcome was actually tied to profit in a way that was beneficial to the patient. That would mean a free market driven by the patient as the consumer could exist. But healthcare systems, especially in the US, generally aren't structured that way.
So, to answer your question about 'why do we get improved, costlier insulin if the old thing was good enough' it is because the healthcare system will make more money on it. If they take a % then they are incentivized to use a more expensive version and they can justify it with the word 'better' even if the person is actually worse off as their financial situation deteriorates and they and their families are forced to cut quality of life everywhere else. They put their line for good enough at the point that makes the most value for them, not the point that is best for the patient.
No, my argument isn't that this wouldn't be used, it is that by using it there will be overage in quality of care above 'good enough' for the same or similar cost. That will result in the most expensive resources being reduced until quality of care is back to 'good enough' at less cost. It isn't a stretch to imagine that a tool like this would lead to a reduction in nursing staff since they can make rounds more effective and now don't need as many people to get the same level of quality job done.