As a specific case, counterfactual donation matches should cause you to donate more, too.
It could be the case that people’s utility functions are pretty sharp near X% of income, so that new information makes little difference. They’re probably directly valuing giving X% of income, perhaps as a personal goal. Some might think that they are spending as much as they want on themselves, and the rest should go to charity.
Or maybe their utility functions just change with new information?