Because there is no such thing as a co pilot type rating, the Yanks do all theirs LHS. At least they did last time I was over there on a Flight Safety conversion course. I was paired with an inexperienced F/O and he found it hard work jumping between seats and duties. For Captain candidates it is a good idea as he gets to know both jobs. So the advantages and disadvantages could be debated forever.
Some airlines in the USA may do it differently, for all I know.