This question would take a whole book to answer properly, but the short answer is because medical insurance companies are caught between the following two problems:
Medical care is expensive, and they make higher profits if they deny as much care as possible to their customers while still charging them high premiums.
If they deny too much care, and too many of their customers get sicker and/or die, people will stop buying their policies. Because all of the companies behave in similar ways, they all worry (correctly) that the public will eventually force the government to change the rules in ways that will allow them less ability to profit.
So the insurance companies have to walk a tightrope, denying as many claims as they think they can get away with but not so many that they look like villains. They actually make the process complicated on purpose, because a certain number of people will just opt to forego health care for non-life-threatening problems rather than go through the excruciating process of filing an insurance claim. Sometimes health care providers and/or their customers will just give up and eat the cost, because they are tired of fighting with the insurance company. So making the claims process complicated saves them money, and thus increases their profit.
Similarly, creating complicated policies that cover certain things but not others gives them an ability to deny care whenever they think they can get away with it. They make complicated deals with various networks of healthcare providers, which limits their customers to only using these particular providers. This allows the insurance companies to gouge these providers and treat them poorly, because they can threaten to yank away a lot of their business if they complain.
There are many more dastardly things these companies do, all in the service of the Great God of Profit. Read Also : Is the US health insurance system essentially a scam?