If it were NPN --
How
would you do it?
The NPN base must be slightly above its emitter voltage. The NPN base must also be above the diff pair collector voltage. Thus the NPN emitter needs to sit on an elevated supply to begin with.
The NPN collector must be above its emitter voltage as well. So the output must be very high up, indeed.
You cannot make a DC coupled, unity gain follower with such a configuration: the voltages are in completely the wrong ranges!
Suppose you wanted to cascade two of these stages (it happens often in analog signal paths, cascading several op-amp circuits, or amplifiers in general). Where do the voltages need to be, now?
Going back to the P/N transformation I mentioned earlier: it can be applied to any single component as well. Reflect on the benefits / challenges this affords the DC voltages and currents in the circuit.
Tim