On average we have progressed tremendously over the last 200 years, say. That means for many diseases, we have made extraordinary progress, as evidenced by life expectancy.
At the same time there are many diseases where we are simply touching the surface and have a very basic (maybe even incorrect) understanding. An example that comes to mind is we still don't know why we sleep. We have good guesses, and we are excellent at observing sleep and curing many related diseases. But why do we spend 1/3 of our lives asleep? We don't know.
In other words, progress is not distributed uniformly over diseases.
(On a different note, the dark ages weren't actually as dark [1].)
Then there is a difference between the life expectancy counted from birth, or counted from 18-20+ yrs up.
People like to think that some X has the perfect answer to any problem we might face, but in fact we don't have answers to very many problems.
The ability to solve various problems is not binary.