Here’s the problem. Let’s say you have a doctor club, where everyone pays the same amount regardless of how often they use the doctor. For people who need the doctor a lot, that’s great. They pay a lot less than they would if they had to pay per visit. For people who just need one checkup a year, they end up paying a lot more than if they just paid for their annual checkup. And they would quickly figure that out, and drop out of the program.

So now the people who are all basically healthy aren’t in your pool anymore. They’re paying for their annual checkup at another doctor. So only the people who need the doctor a lot are paying in. So you have to hire more doctors and increase the cost of the program, because everyone who is in it needs a lot of doctor time.

But then the same thing happens again. People who need more visits a year are getting more out of the program than they are paying in, and people who need fewer visits a year are getting less than they are paying. So the people who need the fewest doctor visits drop out. And so on as the cycle repeats.

You get the idea. There’s a game theory term for this that I am forgetting, but the result is spiraling costs and more dropouts. This is why the ACA (for you non-Americans, that’s the Affordable Care Act, which was attempting to reduce US healthcare costs) had a health insurance mandate. Requiring everyone to be part of the program is the only way to make something like this work.