The whole point of insurance is that there is a relatively low probability of a relatively high expense event happening. In all likelihood, the thing won't happen to you, but if we all pool our money together and you are the unlucky person then you're covered.

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?