Not all treatment or medicine is effective or even beneficial. The Covid shots initially wanted $3000 per shot. Do you want an insurance company that pays for all the dummies that wanted it? Or do you want an insurance company that refuse to pay such experimental meds? The choice obviously has effect on your premium.
The real monopoly is not insurance company, but medicine itself: the licensing requirement for practicing medicine and prescribing meds. These licenses become the choke points creating high prices. If only one store in a town is allowed to sell food, a hamburg would cost $500 and the price would be rising at 20% per year while government employees would be hired to ban backyard chicken.