The most common explanation is that severe anxiety and depression are frequently associated with physical illness symptoms for which no one can find a cause. The pain is real in the sense that it is truly felt, but the cause is mental and not physical. Medication for anxiety and depression often reduces the symptoms.
Additionally, being anxious makes you hypochondriac. I am prone to panic attacks, and they feel like a heart attack - the first time I had one, I called 911 to report a heart attack!
As for the constant pain - pain and its management is an extremely complex and personal battle. Obviously, it looks as if it's all in her head. But even if it is, after all these years, she won't be able to change her outlook anyway, so even if you read her the riot act, it will hurt her feelings and not solve anything.
How does your poor father feel? He deserves a vacation.