Why doesn't natural selection eliminate genetic diseases?

I can think of a few special cases that would explain why a genetic defect is able to persist. I will list them, along with some speculations, to move the discussion forward, but I would like to know if there are explanations for other cases.

The gene that causes sickle cell anemia does so when there are two copies of the gene, but if there is just one, which is much more common, it provides protection against malaria. Are there other genetic defects that follow the same pattern of causing problems if there are two copies but providing protection if there is just one?

Another possibility would be that a disease is of comparatively recent origin. Maybe there are certain mutations that occur relatively often, allowing the perpetuation of the defect.

There is the possibility that what we call a disease was of benefit at an earlier time in our history. There was a recent post that said that the symptoms of ADHD are beneficial for hunter gatherers. What about other mental disorders with a genetic component like depression and schizophrenia?

There are some genetic diseases that do not occur until comparatively late in life. These diseases may not have occurred with any frequency until fairly recently, because people are living longer lives, or maybe after a person successfully raises a family the genetic defect does not have much of an evolutionary impact.

Are there any other explanations? Why, for example, do people get hereditary diabetes or hemophilia?