Because he doesn't understand what's happening with the time loop and just thinks killing Strange will end things.
He thinks that the first couple of times through the loop. He ultimately realizes that it doesn’t work that way, which is why he’s forced to make a deal to end things.
There's nothing that indicates Strange forgot everything.
I think there is, as I outlined in my previous post. And judging by my discussions with other fans and the Headscratchers page on TV Tropes, I’m not alone.
Otherwise he wouldn't know if his plan was working.
He knows there are only two ways it can play out. Either Dormammu eventually makes a deal, or the two of them are stuck in the time loop for all eternity — with only Dormammu perceiving it that way, making Dormammu much more desperate to break out than Strange.
Eventually Dormammu has to realize what’s happening, and there’s no reason he would choose to remain trapped in this purgatory forever. Eventually he has to make a deal. And even if he doesn’t, that means Strange traps Dormammu in eternal torment and saves Earth at the cost of the rest of his own life, a trade he is completely willing to make.
If he took an average of 10 seconds per option, he’d have aged about 4.5 years. Hardly time to shrivel.
10 seconds? Imagine watching Infinity War (or any of 14 million other moves) projected at 24,000 FPS so that the whole movie takes a few seconds. Would you be able to tell me what happened in that movie?
Strange has to watch a lot of events play out in each of these futures. 10 seconds isn’t going to do it.
