Because he isn’t omniscient and he can only be in one place at a time. As evidenced by his inability to stop both the missle heading for Hackinsack, NJ and the San Andreas fault. Which of course makes no sense 5 minutes later as he can now travel faster than the speed of light.
The job of the rest of the Justice League was mainly keeping the kryptonite away from Superman.
If he could fly around the earth hundreds of times a second, how come he couldn’t cross the US in the several minutes between pushing the first missile into space and the second one going off?
In fact, it was established that Silver Age Superman and Superboy could go back in time and not affect it. I remember a story where Superboy went back to save Lincoln, and was thwarted by Luthor happening to have gone back also. I don’t know what the Superman of Earth e ** π can do - they lost me years ago.
And I agree that is one of the stupidest solutions to a movie problem in history.
And they’ve never addressed the effect of getting, say, a grasshopper in the eye while traveling at near light speeds. You know it would happen eventually.
He flew from east to west. when I did that, I time travelled in the opposite direction. I got on an airplane just after sundown on Wednesday night, it was dark for 18 hours then when the sun came up it was Friday morning.
He flies east to west at super-light speed, gets the earth moving backwards. Then he flies around west-to-east a few times to get the earth moving in the right direction again. That implies that he turns back time, then has to correct the flow of time by correcting the direction of the earth’s rotation.