He didn't publicly donate to charitable organizations. Given the kind of private person he was, it's likely that he preferred to donate anonymously.
It's not uncommon that someone would reject the idea of getting credit for doing the right thing.
!!!!!! New York Times published about this. Steve was never the one for glossy press when it comes to his personal life, including his fortune. Just because you don't see the transaction, doesn't mean it does not exist.