Tony didn’t save the world because it was in his best interest to do so.
The five years that followed Avengers: Infinity War was hell for some, but for Tony, there were upsides.
Tony swiped the Infinity Stones from Thanos at the last moment and gave his life to defeat the villain.
Despite that, the hero knew there would only be one chance to defeat Thanos, and when the moment came, he didn’t hesitate to save the world when he had to.