Question normal

My name is Devvrat Malhotra; I am a MD Candidate at UCSF and MPH candidate at Harvard School of Public Health. In our private health care market, insurance companies and medicare are buyers and pharmaceutical companies are the suppliers. I do not understand why insurance companies agree to pay exuberant cost for drugs such as Harvoni for Hep C. In a normal economic model, I would anticipate that buyers would balk at such ridiculous prices. I have heard some opinions around the medicare law from Bush era that prevents medicare from bargaining but why not have an insurance company consortium that negotiates drug prices? Thank you for your responses.