Do you think that big pharmaceutical companies would reduce prices if everything they produced would be a guaranteed sale to governments? I don't.
Pharmaceutical companies have a disincentive to develop medicines which cure diseases. There's a lot more money to be made from medicine which manages symptoms.
There are former opiate addicts who haven't used heroin/ morphine, etc for 30+ years who still drink methadone daily, at the taxpayers' expense. They should have been weaned years ago, by gradually reducing and/ or diluting the methadone until it was no longer required, but that would end mean a decline in methadone sales, resulting in a smaller dividend for shareholders.
Pharmaceutical companies also create and sell non-essential medicines. Should the taxpayer foot the bill for his neighbour's Cialis, or a co-worker's Minoxidil? If all health care were free, would things like liposuction, nose jobs or lip implants become rights for the people who want them?