It's an investment.

When you have some asset that you don't immediately need, and somebody else would be able to make better use of it, renting it out or giving it away enriches the whole system. Then you get to live in that richer system and enjoy the benefits. Your quality of life is better when you live somewhere with reliable electricity, uncensored internet, and trivial access to potable water, right? A fancy car isn't much fun without fuel or decent roads.

Helping somebody on an altruistic basis is just another transfer of resources toward where they'll be used more efficiently. It's less directly profitable to the donor than sale or rent, but reduced transaction costs and targeting explicitly based on need means the net societal benefit can be greater.

Maximizing overall QALYs may be, in itself, a less efficient way to improve the society you live in than slanting toward helping your immediate social circle, municipality, or nation, but it's easier for everyone to agree on, and every dollar or man-hour spent on arguments is one less to spend on getting the actual work done. Besides, we live in a world where more of the mercury contaminating fish in Lake Michigan comes from industry in China than from local sources. You never know whose problems might land in your back yard, so just go ahead and solve all of them.

All that social stuff, instinctive empathy and cultural expectations alike, is secondary. It developed so people can do the right thing without needing to understand why it's the right thing.