I wonder if it's economies of scale. If you charge £10 for AYCE and someone just takes a roll and coffee, you're still quids in. If you have a family where one person wants the full monty but the other one doesn't, then you'd be hard pressed to run a separate accounting/server order system when the two people would feel obliged to pay the same anyway regardless of quantity. Cruise ships have a policy whereby two people sharing the same stateroom must buy the same drinks package (if one person is having alcohol, the other has to, which really penalises a couple I know, one of whom drinks a fair amount and one who only really drinks water, and the kicker is that bottled water may not even be on the soft drink package either) because otherwise one could simply subsidise the other without them chipping in to what is probably the main revenue generator on board.
As someone who takes notes for a group of management accountants where our customer has got away for a long time running up big bills while being subsidised by another org (all public sector -- not hospitality, but think of it like someone taking advantage of a hotel buffet to stock up for breakfast, lunch and dinner) and generates an insane number of demands knowing that they won't ultimately be picking up the tab, these decisions are made on a very granular basis. They probably gain more from couples who have different habits than from me taking up colleague time ordering very small amounts. The solution is to enforce policies that don't allow so-called 'whales' to take advantage -- like stopping people taking food out of the cafeteria etc -- rather than just add more granular costs to the system.
More frustrating about low-frills hotels is the lack of bottled water and biscuits in the rooms, but again, it's worth it I suppose to keep the costs down so I have the essentials like a warm place to sleep.
The thing that Travelodge etc get right is a free coffee machine in the bar. It's machine coffee, but on a cold night when you just want something wet and warm it's a godsend.