They're priced this way because they're 80-90% cheaper than the previous treatment.
Previously, with no cure, HepC patients would have eventually had a liver transplant. Liver transplants cost between $500K-$1M. Of course, it also requires a liver, so many would die waiting for a liver.
As you noted, the drugs have become much cheaper. Mavyret is ~1/2 the cost of Sovaldi, the first drug on the market (replaced by Harvoni). Mavyret actually only has an 8 week treatment, which is an improvement over the 12 weeks of Harvoni.
The competition within these drugs became so fierce that, despite these prices, no new ones are planned (last I read). In fact, one you listed here has been off the market for over a year
https://www.empr.com/home/news/two-hepatitis-c-virus-infecti...