from an evolutionary psychology standpoint, we are all driven to constantly increase our social value (and indirectly that of our offspring). once you hit a certain level of wealth, it is rational for you to shift your attention from making more money. your social value is only increased by wealth up to a certain amount, after which the marginal returns in social capital approach zero for every additional dollar earned. to put it simply, how much more social value do you have with $1 billion versus $2 billion? The world is indifferent. At that point, you are simply a rich person.
once you reach such an inflection point of wealth, you must find an alternative means of increasing your social capital. there are two common ways: one is to demonstrate your evolutionary fitness in a field completely unrelated to how you amassed your wealth: many rich get into movie production, novel writing or other creative endeavor. These usually fail.
another alternative, the safer alternative, is to expend your remaining time and resources advancing social causes (running for an elected office certainly falls under this category). this is the simplest way for an adult, untrained in anything other than their primary business, to increase their social capital.
it's not a coincidence that zuck is still willing $450mm to his offspring--more than enough to hit that inflection point. he knows that any additional dollar beyond that inflection point provides marginal social value to his offspring.