Even billionaires are not as mobile as they like to pretend. Yes, they certainly have the means to pull up and relocate to some private island in the tropics where they won't be taxed, if they really wanted. However, by and large, their wealth is maintained by access to markets, connections and institutions without which, they canot function as well, and places like New York City have these things in droves. They don't want to pay 1% more in taxes a year if they can get away with it by bluffing, but do you really think they'll let it go if they still make more money in NY after raising taxes on them than they would in their private island?