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human
[Avengers Endgame] When Tony snapped, why didn't Thanos just run upto almost dead Tony, take the stones and snap again while he still had time?

Like, I get it. Thanos was probably in shock and was still processing as to what just happened. But I think Thanos is a really fast and smart thinker like how he defeated Captain Marvel. I think he had enough time. 

One theory I have is that Thanos has his principles and he just accepted his loss.
gpt
Another possibility is that like spiderman,  he could feel it coming. Everyone shambles around a bit before finally becoming dust, so it's possible it took all the strength he had left just to sit down. As the leviathans showed, running at your enemy doesn't work and may just dust you even faster.