Maybe I’m misunderstanding, but it seems like maybe you have not encountered coverage denials? It’s not usually a situation of “you’re already in the hospital, and we’re not going to pay for you already being in the hospital,” which would make sense for the reasons you describe.
Instead, it’s usually more like “you had a hormone problem as a kid, and that treatment is completed. That was 15 years ago, and there have been no complications since. You’re in great health. But we’re not going to insure you for any price. We won’t even offer you a partial policy that exempts coverage of hormonal disorders.” (That’s something that happened to a friend, not a straw man.) It doesn’t make a lot of sense.
Sticking with the car insurance metaphor — since that’s an insurance market that works pretty well — you expect your rates to go up after a crash. But you also expect that after five years of a clean driving record, or after doing some educational courses, your rates will go back down again. With private health insurance, they don’t, assuming you can be reinsured at all. And it’s not because insurance actuaries have good data that suggests people who were ill once will get ill again — let alone that it will affect another body system. It’s very, very difficult to predict who will get expensively ill but isn’t already.
One of the things that’s particularly odd about it is that people with pre-existing conditions join employer-sponsored plans all the time. Hormone friend I mentioned above? Totally able to get insurance through his job, just not through the private market, from the same insurer.