Atheists make up 10% of the population (allegedly), so you're going to have to explain people like Warren Buffet and Bill Gates. And Andrew Carnegie. And George Soros. And Ted Turner (who is admittedly a kook). Also DWB/MSF and similar organizations. Proportionally speaking, we have you beat by a long shot.
Assuming you're making who is the most charitable a contest.
If someone throws you a surprise party, is it okay to stay mad at them for having forgotten your birthday even after everyone yells, "SURPRISE!" and we're all eating birthday cake and drinking champagne?
Hint: finish reading my OP and follow the link, Mak old buddy.
Hold on, I just can't leave this alone. Let me try one:
The reason atheists folk don't donate is because the existentialist angst that plagues them each numbered day pretty much jackhammers into the chemical reaction they call a conscience: "What's the use?"