"There's no reason why the 1, 30, 31, 36, 62, 63 etc need to go anywhere near the centre where pedestrians are and could avoid it via the Central Motorway / Central Station."

Because very few passengers are travelling the full route? People are catching them from their respective sides of the city to get in and out of the city centre. Saying these services don't need to stop in the city centre is like suggesting the X21 doesn't bother with Durham and Chester Le street.