The thing about health insurance is that it isn't really insurance in the traditional sense.
Traditional insurance is something like "just in case something bad happens while you are covered, we will pay for it."
But that's not how our healthcare system works. People get sick while they are unemployed, they get sick while they are employed, they may stay sick after losing their job. Healthcare costs are exceptionally expensive, so it is unrealistic to expect ANYBODY short of multi-millionaires to be able to afford to cover their own healthcare costs in the event of a serious illness. Therefore the only solution is socialism, and our socialist system is referred to as "health insurance". Health insurance MUST be structured such that everybody, healthy and sick, pays into the system to one extent or another through premiums, so that there are enough cash reserves in the system to pay for the care of people who get seriously ill.
The alternative would be that everybody who isn't EXTREMELY rich has to lead their life in fear that if they ever get cancer they will have to guaranteed to die through lack of care, but not until they first go bankrupt exhausting all of their financial resources. Even if they do have a few million in the bank due to hard work and careful savings, it might not be enough.
Society at large has decided that socialism or health insurance (which is rather socialist in nature) is vastly preferable to the alternative.
Now the fact that the medical industry is corrupt and dramatically overcharges for things that could be cheap, just because the insurance company is paying and can afford it... that's a serious matter that should be dealt with somehow.
My point is, denying coverage to people with pre-existing conditions is ludicrous in a world where insurance is necessary for survival, but not everyone who is capable and hardworking can always have insurance, all it takes is a few months of unemployment in a tough economy. But then when they get a job with good coverage... sorry, you got sick in between jobs, we'll just let you die!