Why doesn't the American market provide efficient and effective health insurance like it does for car insurance?

Car insurance is relatively simple. I shop around, telling them how much coverage I want. They request my driving history, and give me a quote. At any time, I can shop around and change insurance policies without any problems. Once it's time to collect payment, it's a relatively simple matter. What makes health insurance so difficult, controlling, unreliable, and expensive? For example, with health insurance:

• Can only shop during a specific enrollment period

• Policies are so complex, the vast majority of the population can't understand them

• It's commonly provided in part by the employer because buying a policy otherwise is prohibitively expensive

• Insurance companies are notorious for denying payments