I wondered that, and came to the conclusion (a few years ago) that it was because then you wouldn't go into the kiosk and make all those impulse chocolate/screenwash/chamois/road atlas purchases.
Now that some petrol stations have a pay at the pump card facility, people don't have to go in anyway, and it might make more sense - after all, the driver would never have to get out into the wet/cold, get their hands 'dirty', etc.
With hand held card terminals, or terminals attached to the pump, it should be easy. And drivers wanting anything from the shop could go in anyway while the attendant does the fuel.