You are right...
Expression Columns don't show up in Customize View.
... and wrong.
I knew that before this thread existed. It just tried to explain how it currently works.
As I said, MC's tools cannot access Expression Columns because they are not library fields, be it right or wrong. Customize View is one of these tools. When the Expression Column feature was added (it's a relatively new thing) the view configuration system was not redesigned to be able to handle them. Perhaps "Columns to show" should be renamed as "Library fields to show as columns" or something like that to avoid confusion.
Nevertheless, I don't disagree with you. It would be nice if Customize View would show also Expression Columns. Then it would be possible to copy existing Expression Columns to a new view by using the Save/Load View feature.
EDIT
I have a good reason to believe that you are going to like at least one of the new features in the next build.