You have to weigh that against the fact that you are much more able to figure out how to actually do what's needed at levels where you see things firsthand. At least, that's been my experience; it's much more realistic to start a nonprofit that can make a real difference locally, then perhaps scale with time, than it is to found something with a global mission, lacking global context on how things manifest around the world.

More importantly, I'm not a utilitarian, and do not subscribe to "effective altruism" or other utilitarian philosophies. At the end of the day it's Gates' money to do with as he wishes and it's my internet account to argue against that as I wish.