Same reason who witness who says I do not know of any party I was not here
which is Ford is so traumatized no one would remember except for her.
That being said, anyone who would attempt to write a letter such as this alleged 35 year old incident, which they had old notes of details regarding it, would have had those documented details right next to them for reference, as they wrote the letter.
If she suddenly has or fabricates notes and a calendar such as Kavanaugh does as you inferred, I really hope she uses that excuse you mentioned. That alone would impeach this individual, in my opinion of course!