impossible to have a joinfield from custom object 9 to get a field value coming from parent activity

Hi,I don't get it please help me. You can create n CO9 records for a unique parent activity and see them in the CO9 related section but you can't create a joinfield thru workflow because there is no "parent activity id" field in the C09 field definition. Why can't it work just like when you use a joinfield to populate a contact record with values coming from its parent account?
Also how come when you click one of the child CO9 records it does not show anywhere (header or related section) its parent activity?
I am sorry this does not make sense at all to me especially when you can report both directions (parent to see children, child to see parent)