UPDATE: The scope of this ticket was reduced to optional areas only. Please see the comment for the reason why and a work-around.
Unlike components, page and area nodes are created and edited in two distinct steps, so at the moment we don't know when they are actually new, and thus don't apply default values (regardless of field-type).
This may be a custom behavior to implement specifically in the pages app.
e.g. using an OptionGroupFieldDefinition and setting default option with the "selected" property:
If dialog is called from component, it is working. To reproduce: Copy a OptionGroup e.g. from Form Module to a Area Dialog. No Value is selected.