Because the relationship between money and utility is not constant across the human population. If we consider a meal at a moderately fancy resteraunt then the

Because the relationship between money and utility is not constant across the human population. If we consider a meal at a moderately fancy resteraunt then there are people without much money who would be aghast at spending that much on a meal. Conversely there are rich people who wouldn't even look at the menu-prices or the bill.