Locals will know their way around, have less luggage and be dispersing. How many need to cross the city centre rather than go somewhere in between?
Better than having a group of non-locals ambling confused around a station clogging up lifts and stairs with suitcases.
I wonder how everyone manages on their way to Heathrow and Gatwick. Though using the logic applied on this forum, I'm surprised we haven't seen demands for the LU subsurface lines to be repurposed to allow direct services to London's airports from Banbury or Norwich