"I have asked this question many times why the first officer is not allowed to taxi and have never received an acceptable answer".
Ask the training/management departments of the relevant airline why they've deleted another piece of fact, logic and reasoning from the profession.
Alas many 'frames don't have a tiller on the RHS, presumably due to the penny-pinching when they were originally specified with the maker. Boeing 73s & NGs in Europe are but one example.
However, there's nothing to stop competent captains from allowing the F/O to "drive" these 'frames on straight stretches of taxiway with shallow turns to give them confidence, so that in the event of landing with a dead captain they could at least steer the beast off onto the highspeed exit (where supplied) and save the airport operator the hassle of a blocked runway.
All the F/Os to whom I gave the aforesaid taxi practice were always very pleased and happy to practise speed control on the ground and rudder fine steering. Of course I had to ask them not to blab about it in case the headmaster got to know of it and use the event to rap my knuckles or worse, such is the apalling environment in which we operate(d).
