Centaurus,
it's got nothing to do with ability (or competency) and everything to do with regulation. It is the CAA (and it's overseas equivalents) that dictate who can operate from which seat. It is the same reason that Captains have to undergo a 'Right Hand Seat Check' once a year in the sim to enable them to operate as an FO if required.
As FO's don't undergo a 'Left Hand Seat Check' (at least until they get a command!) they are not 'qualified' in the regulatory sense to operate from that seat.
I can't think of any Long Haul First officer who would not be able to fly the aircraft just as competently from the LHS as from the right. It's simply that we are not *legally* allowed to.
HTH!