Not a great idea for the taxiing operator to be looking at charts too. Just the time that errant vehicle or animal to cross.
Same reason for Captains taxiing in some airlines. Captain has final responsibility for safety of aircraft. If he is head down in charts temporarily lost in taxiway maize, while FO clips the wingtip on something.... Or let's the mains go cross country - Who carries the can?
Not everyone operates in environment of 'same nationality' crew with easy communication. (I.e. Good English) This growing communication problem is one of the invisible safety factors increasing in aviation these days.
And finally the even better reason..... Generally - Younger FO's have better eyes for reading tiny print on Jepp plates!
