I know that configured default values in field definitions are not saved to database, they are only used to preset a field's value in the GUI
But why is this the case? Also in #3853 (comment) this fact is cited - but what is the reason for this?

By not saving default values to the database a lot of problems arise, e.g. the value does not appear in exports. When you then open the object to check if the value really has not been set, it gets even more confusing because there the default value gets applied.