That's the difference between the DC and Marvel Universe. For the most part Marvel's superheroes let everyone one know who they are. In the DC universe the secret identity seems much more important.
Actually that's a really new development. It used to be the other way around.
The older I get the harder it is for me to suspend disbelief that people don't know Clark Kent is Superman.
That's one of the reasons what Reeve did was so remarkable.
bear in mind, too that when Superman was created, there weren't high speed camera taking pictures of absolutely everything and everyone at every second.
Though, of course, Lois was always in close proximity to him. She just was never very bright.