In the aerospace industry, to get that last percent of a percent, 2 completely independent implementations of everything are used. Then to get another decimal, you add 2 more implementations and a consensus algorithm. Then of course you add static/unit/api/integration/stress/fuzz test suits for each implementation. Then test the tests. Then have a human run each test as the "second implementation" of the CI system. And so on, and so on. Each new decimal "9" cost multiple time more in human resource alone.

Then take into account the "productivity loss" of all those process and you need yet more poeple to progress as fast. Adding more people to a project has a diminishing return. After a while you can spend the entire GDP of the world and you wont be able to add another availability decimal point.