If Denzel Washington can play a villain (Training Day) or a misdirected protagonist (Flight), why can't Tom Hanks do so? (Maybe he has--I haven't seen every Tom Hanks movie.)

The role of a "nice guy" actor (Hanks, Stewart) may be to create a myth that there's something inherently nice about white American men. Other heroic actors (John Wayne in Red River or The Searchers, for example) occasionally play against type (Henry Fonda especially in Once Upon a Time in America), but Hanks and Stewart don't tend to (maybe Stewart in Vertigo).