Same reason you can't copy/paste a standard UI window title bar text.
If you need to copy/paste some text, perhaps a label or textfield is the right UI widget to use.
The title bar in the contentpanel is not designed to be copy/pasted from and I believe all mouse events are being consumed to handle other actions (like drag/drop/resize/click etc).
In GXT, all components can have text selection disabled using disableTextSelection. The header in a content panel disables text selection. You can call panel.getHeader().disableTextSelection(false). However, this must be called after the panel is rendered as content panel sets the value when it is rendered.