From my perspective, it's "I have to think about all the problems in the world and care about them." That's burdensome. So instead I look vaguely around for 100% solutions to these problems, things where I don't actually need to think about people currently suffering (as I would in order to determine how effective incremental solutions are), things sufficiently nebulous and far-in-the-future that I don't have to worry about connecting them to people starving in distant lands.