Let's say your odds of getting that cancer are 5% and you know that ahead of time. So, knowing that it would bankrupt you, you gather 20 other people who are also at a 5% risk of getting cancer to form a group where you all chip in no matter who gets sick. You've now spread the risk out across a pool of people and, yet, everyone is still paying their fair share because no one knows who will be the unfortunate cancer patient at the time the agreement is made.
But in your little group, there's still a 36% that no one will get cancer and that's offset by a small chance that multiple members of you group will get the disease. If even 4/20 members get cancer, that would now come pretty close to bankrupting everyone in the group. So what do you do? You increase the size of the group and lower the chances that no one will get cancer while also increasing the chances that only a proportional number of people (5%) in the group get the disease.
But then along comes Danny. We also know that Danny has a 10% chance of getting cancer. He also wants to join our group. If we just let him join, it won't be fair...after all he's twice as likely to need us all to pay for him. So we can either tell him to get lost, or we can just make sure that when the bills for someone's cancer show up, he pays twice as much as any other member. He's bringing twice the risk with him so he pays twice as much.
In poker, players learn to avoid being outcome-oriented when evaluating their decision making. Rather than looking at a decision they made and seeing whether they won or lost, it's more important to look at the decision and ask themselves if they'd made the same decision a million times, how many times would it lose and how many times would it win. Because any individual hand where a correct decision is made can still lose due to variance.
Insurance is the same concept. Actuaries use statistical analysis to determine an individual's risk so insurance companies can price that risk accordingly. Over a million lifetimes, your insurance premiums (less the insurance company's profits) would come very close to the overall insurance payouts. But since you've only got the one life, you'll be under- or over-paying. But you're still only paying for your own risk, you're not paying for anyone else's risk.
Where the ACA comes in is when Danny can't really afford the double stake. It forces us to let him join the group and pay less than double whenever someone gets cancer. That's a bad deal for all of us 5%ers, despite the fact that there's still a 90% chance that Danny doesn't get sick.
Now try to answer your question...do you see the difference between paying for your own portion of the risk pool vs lower risk individuals subsidizing higher risk individuals? If you were in one of those groups of 5%ers, would you just let Danny join without adjusting for his added risk or would you be looking for more 5%ers like yourself?