I assume it is because "thickness" is a system defined global variable, used for sheet metal. This is exactly what I want. There is an old hack (in previous versions of SolidWorks), where if you define "thickness" once, it then always brings up the option to tie the thickness of any extude to the thickness variable.
Is there a new work-around in more recent versions of SolidWorks? I haven't tried to do this in quite a while.
No Charles, there isn't. I noticed that too. I've also observed that it's a royal pain to link custom properties and dimensions to this value because the dimension associated with the sheet metal thickness doesn't show up when you double-click the base-flange or sheetmetal feature-only when you turn on all feature dimensions.