The reasoning being that an FO in the LHS most likely has no experience of handling that aircraft type from the LHS. Makes sense.
Surely each pilot in these airline transports should be capable and competent at monitoring the automatic pilot from either seat. And also that each pilot should be competent at hand flying from either seat. After all the seats are close together and the thrust levers are easy to reach from either position. And both pilots have identical flight instrument displays. Of course if all the crew do not have the basic hand flying competency then everyone is in serious trouble if hand flying is suddenly needed.