In insurance, people who get lucky subsidize people who get unlucky. If you can test luck before getting insurance, then lucky people don't get insurance and there's nobody left to subsidize unlucky people.

> Forbidding insurance companies to discriminate based on luck does not solve this problem, because the problem is caused by customers selectively buying insurance based on their own luck.

It's a little more nuanced than that. Let's say men are more prone to car crashes than women. Either you charge men more for auto insurance, OR you charge everyone a flat rate, and the market will rapidly clear itself of all women, since that flat rate will be too high to appeal to women. In this case, you end up with a market that's exclusively men, and women are uninsured.

When applied to health insurance, this means that, if you're forcing companies to insure everyone, they will have to insure people predisposed to expensive illnesses at incredibly high rates, because that's the expected cost of their lifetime care.

One implication of this is that the "no discrimination for pre-existing conditions" portion of the ACA is equivalent to "if you have a pre-existing condition, your coverage will be exorbitantly expensive".

There are ways of hiding this extra cost, but at the end of the day, it's like sweeping dust under the rug: it all has to sum to zero.