Also, the only thing stopping medical insurers from denying coverage or increasing prices for people with pre-existing conditions is the Affordable Care Act.
You can get insurance on very risky things. The way it's done is high rates and "reinsurance" which means the insurance company shares the risk (and premium) with a pool of other insurance companies. For policies with a large possible payout (e.g. a big ship or aircraft) this is always done, to avoid issues for the insurance company that gets unlucky enough to have to pay out.
A pre-existing condition is not a low probability event. It is a 100% certainty that you have to pay for it. In that case the insurance is just a middleman who is increasing the cost of the thing you definitely have to pay for. You pay the insurance company, they take a cut, and they pay the healthcare provider. Why not just pay the healthcare provider directly?
Second of all: when you take out an insurance policy and pay your first premium, you are immediately covered. If you are "self insuring" by setting aside the same amount monthly, it will take you a long time to even begin to accumulate enough funds to cover yourself in case of catastrophe. (And due to the first point, you may never reach that point.)
Finally, yes, of course, there is no need to take out an insurance policy for a risk that you can afford to pay out-of-pocket. It's like how if you have the cash to pay for a house, there is no need to take out a mortgage.
