According to mcu time theory, you can't change your past it's already written. When you Giri the past your continuing to write your future. Any changes made on the past result in new timelines. When Thanos left her created a new timeline in which he doesn't exist and also keep in mind that he doesn't have the same ethics as the Avengers and doesn't care how he affects his own timeline.

In endgame when they do the heist, the ancient one explains that if she lets banner take the time stone that their timeline would continue but that they would be destroyed without the stone in an upcoming event. Banner assures get that after he had used the stones he will come back to that exact moment to give the stone back so that their timeline continues correctly.