"Why can't the government just pass laws, forcing the HMOs to give me the best possible care?"
So much of medical care responsibility is a gray issue.  Should you be entitled to experimental drugs, which haven't been properly tested and may be prohibitively expensive, and may not benefit you at all?  Is this higher-priced drug really going to give you any additional benefit?  Experimental surgery can cost hundreds of thousands, or millions of dollars; will it give you a chance, or be money wasted?
A medical insurer with no stake in whether you live or die, or slowly waste away, will use the most conservative interpretation, and save themselves some money.  Pass laws forcing HMOs to use the best possible care, and you will see monthly costs skyrocket.  Restrict the fees HMOs can charge too, and you will only be forcing them out of business.
If you're considering letting the government take over medical care, take a good look at Canada.  Their already high tax rates can't be increased enough to support the dreadful public health system, which makes people wait months or years for diagnostics and treatment.
When you buy so much insurance from your HMO, they earn more money.  By keeping you alive and healthy longer, they can earn still more; so it would make sense for them to invest in medical research.  An insurer who doesn't have a direct interest in your health would not benefit by furthering medical research, and might benefit more by hindering it.