The proplem is that once a person gets into a good airline, a major carrier, they tend to stay there. Movement is by "Dead Mens Shoes".
If this same carrier was to accept direct entry commands the F/Os would sit for a hell of a long time in the right seat.
Maybe the solution to this would be to promote F/Os, depending on ability and hours, to command whether they were required for the position or not. Flights would then be rostered with two Captains flying leg for leg, left seat right seat.