Originally Posted by usmc-sgt

This is the reason. It’s the same plane, but requires a bit of currency and regularity to maintain proficiency. There are just enough logistical pieces and requirements where it would be less than ideal for someone to be qualified and then two years later do an ETOPS trip.

Any pilot can do it as it’s not difficult. It is however unique and different enough where it can’t be done passively in an occasional pairing.