As typically publicly traded companies, they have a fiduciary responsibility to their shareholders to maximize profits. And the only way to maximize profits is to deny coverage.
The purpose and value of insurance is amortizing risk. There are very few things that I think should be controlled by the federal government, but a single payer health care system is one of the few that actually makes sense from an incentives perspective.
In the words of Charlie Munger who's spent more time thinking about insurance than almost anyone: "Show me the incentive and I will show you the outcome."
Why does the US pay the highest prices for mediocre health care? Perverse incentives.