The pilot flying has to do the cockpit checks and set up the FMGC for the sector. There really isn't time for him to the walk around as well while the F/O twiddles his thumbs.
An interesting view of SOPs and quite unreal in most airlines - I obviously did it all wrong for the 20+years - and I never sat there as Captain 'twiddling my thumbs' while the F/O did HIS walk-round
. Are you really an ATPL/737?
Edit: Do you mean FMCG?