Especially since BND (or the predecessor "Operation Gehlen") was established to large parts due to American interest in German spying experience regarding the Soviet Union.
I think its more that the German intelligence services are not liable to the German sovereign, but are rather still 'occupied' by the USA's own military/intelligence agencies.
Well, also, (amply justified by Germany's history) public distrust of mass surveillance. Joining wouldn't really be politically viable.