When cards are connected to an activity it is not possible to cluster this activity resp. to make it the superior activity.
Only "empty" Activities (no connection to cards) can be used as superior activity...makes no sense for me...
We consider the parent activities to be only containers of work to be done, including aggregations of this work. Keeping this strategy makes it logical to keep as it is and to only allow cards to be attached to activities on the lowest level.