To get a little bit back on the OP’s topic, at the scale of “charity” practiced by Gates and Buffett, the distinction between “charity” and “investment” becomes meaningless. Gates and Buffett are not driving around Third World countries throwing $20 bills out the windows of a Land Rover; they’re using money to invest in large scale human and physical resource projects.
How is an immunization or a clean water program not an investment? Immunization programs do, in fact, employ lots of people; the needles and vaccines don’t just appear from thin air and get injected by needle fairies. The program reduces disease, especially childhood disease, thereby increasing health, reducing pressure on the systems that treat disease, and enabling people to be more productive AND spend less money mitigating the effects of treatable illnesses.