Thanos should have planned better.
At the conclusion of Avengers: Infinity War, Thanos snapped his fingers and killed half of life in the galaxy. Although slaughtering billions seems like enough murder, Thanos should have taken down heroes like Tony Stark or Thor if he planned on winning. The Mad Titan may have won the battle, but as Doctor Strange saw, the war is far from over.
In Avengers: Infinity War-The Art of the Movie, Marvel Studios head of visual development Ryan Meinerding explains why Thanos didn't kill all the heroes. "The plan of getting all the Infinity Stones is putting him in a position of being so powerful that he is sort of calmer, more single-minded, and more reasonable," he explained. "He is not necessarily worried about killing the heroes. As long as he ends up with the stones, he can accomplish what he wants. That was sort of the place where we were starting. We were trying to find looks that were more casual, because he wouldn't really need his armor after he had gathered more than one Infinity Stone."
A much easier and more obvious reason is that Marvel plans to make another two billion dollars when they drop Avengers 4, which touches down next Spring.