

Well, first of all, apparently this Superman, while still being the strongest hero, is pretty nerfed compared to his comic version.
Secondly, you have the same problem with the MCU. Why didn’t the Eternals intervene against Thanos? Why wasn’t Captain Marvel called when Loki attacked New York with a Chitauri army? Why did Hawkeye and Black Widow not immediately get Iron Man armours when joining the Avengers? And so on. The answer is of course, it’s a comic book movie. Don’t overthink it.
Which was very likely inspired by scientific accounts of the physical effects of radiation that Lovecraft had read.