Williams: the most successful career of anyone in cinema history?
In terms of the box office success of the films he's participated in, the extremely long length of his career, the extent to which his work is known and loved by the public, and the number of award wins and nominations. While it's true he's "lost" more Oscars than anybody else, 5 Oscars is still quite a lot for a single individual. He will also get a Lifetime Achievement Award, although maybe not during his lifetime.
I think being nominated for more than one film per year several times in his career cut down on some wins, but I think even more Hollywood is uncomfortable being shown up by a composer.