"how can people care so little about the destitution of so much of the world's population?"

The U.S. cared so much about the Dominican Republic and Vietnam and Iraq that it invaded all of them. The U.S. gives Israel more than $3 billion a year so that it can expand settlements in the West Bank. A lot of financial aid ends up in the pockets of corrupt tyrants.

A lot of people might be better off if we cared less. "To care is painful and costly" — and also, sometimes, destructive.