There is plenty of research demonstrating adding money to low income people does increase happiness, but adding money to high income earners does not. So, the question is why does adding money to high income earners not increase happiness. It's generally assumed that high income earners are not spending their disposable income 'optimally'.

PS: Sure, perhaps their not measuring 'happiness' and if it makes you feel better call it fruitkerfluffle if you want. However, that does not invalidate the question.