Because a modern day audience would TOTALLY accept that level of cheese and camp in a 21st Century movie.
First of all, the 1978 movie Superman
wasn't really all that campy. The filmmakers knew that, in order for late-70's audiences to accept Superman, that they had to write the character of Louis Lane as a worldly, skeptical, and cynical adult in order to act as a buffer between Superman and the audiences. By winning Louis over to Superman's "boy-scout" personality, the filmmakers figured they would also win over the equally worldly, skeptical, and cynical adults sitting in the theaters watching the movie. And it worked! Audiences love it!
Secondly, audiences of the late 70's aren't really that different from modern audiences. They are just as philosophically and ideologically "modern" as one another. (One decade simply lacks the smartphones and microchips of the other. That's all.) Audiences of the late 70's also enjoyed Star Wars
(1977) and Alien
(1979), which are two franchises that are still alive and well today without there being any real changes to the narrative approach to the basic stories being told in their respective franchises. The only franchise from the late 70's that changed in more recent films was Superman, and that's because, for whatever reason, contemporary filmmakers thought that audiences that were "modern" enough to enjoy Star Wars were somehow "too old-fashioned" to be relied upon for their enjoyment of Superman. It's a form of thinking that has multiple internal contradictions and doesn't survive logical scrutiny of any kind. Simple as that. If Superman is too old-fashioned for contemporary audiences, then the Star Wars movie that came out before
Superman is also too old-fashioned for contemporary audiences.
And finally, it's not like those darker and more grim Superman movies are doing very well with audiences that enjoyed other late-70's film franchises, such as Star Wars. Maybe somebody should fix that.
In conclusion, saying that modern audiences wouldn't appreciate a real Superman movie isn't logically sound with the rest of the filmmaking landscape in which we currently live. Audiences claiming that they don't like Superman come off as Andy Samberg screaming "I'M AN ADULT" while throwing cake and candy to the ground.
It's one of the reason why we can't have nice things anymore. (To be fair, Star Wars probably isn't marketed to those kinds of people either.)