I think it's a matter of reach. Most ordinary people's good deeds have a small audience—the 'N' in a 1:N spread is limited. But the bigger the fame, the larger the audience becomes. From that perspective, when Keanu Reeves does something good, the number of people influenced by that act is much larger. I believe humans are creatures shaped by their environment—and they are influenced by it. So it's socially acknowledged that piling up these good actions is beneficial, and the greater the influence of such actions, the larger the impact they have. Of course, this is just my hypothesis.