It's not accurate to say the gov't doesn't use it's bargaining power to get lower priced drugs. You are correct that it doesn't negotiate lower prices like the DoD does (through an RFP and bid system), however it does:
1. force drug makers to match the lowest price that it offers to anyone else. In other words Medicare gets the same (or lower) price as big insurers like United.
2. force drug manufacturers to discount the cost of drugs for Medicare Part-D once the patient enters the donut hole. Over the next 5 years that discount will increase to 50% of the cost incurred within the donut hole (this will total several billions per year)
3. for Medicaid, force the drug manufacturers to provide a 23.1% discount off the WAC (wholesale price) OR the lowest price offered to anyone else (whichever is lower)
4. for disproportionate share hospitals (hospitals that serve the uninsured), force drug companies to provide the Medicaid discount (at least 23.1%) to those hospitals for all the drugs they buy, whether or not they go to the uninsured
5. if the drug company increases the cost of its drug at a rate greater than the rate of inflation (CPI), they have to pay CPI penalties each year. There are some drugs that Medicaid gets a 90% discount on (far lower than any insurance company could get)
So yes, you are correct that federal gov't doesn't directly negotiate individual drug prices, but it sure as heck uses it bargaining power to get some super sweet deals!