The only reason The Avengers got screwed and
had to die was because they
sent Nebula back in time.
At the end of Avengers: Infinity War, why didn't Doctor Strange simply tell Tony to not send Nebula back in time?
As Doctor Strange says himself they only win in one scenario, the one we see, for that to happen things had to play out exactly as we saw them. If they didn't then they wouldn't win.
Doctor Strange: I went forward in time to view alternate futures. To see all the possible outcomes of the coming conflict.
Peter Quill: How many did you see?
Doctor Strange: 14,000,605.
Tony Stark: How many did we win?
Doctor Strange: One.
Avengers: Infinity War
Then in Endgame Strange refuses to tell Stark if they're in the winning outcome telling him that if he does it won't happen. As such we can assume that he couldn't tell Tony that on Titan for the same reason... which is true had Nebula not gone back things wouldn't have happened the way they did.
Tony Stark: You said one out of 14 million, we win, yeah? Tell me this is it.
Doctor Strange: If I tell you what happens, it won't happen.