Because video games have a hard time doing the things that actually makes Superman, well, Superman?
Video games can do his powers, easily. Superspeed has been done, super strength, invulnerability, heat vision, frost breath, super senses. All have some kinds of precedent in video games already. But if you just make a video game with a character that has those powers, it will be boring. If you want Superman, you have to focus on the man.
Consider this for a Superman moment: Superman is hovering in the sky above Metropolis, he's barely moving, and looks alert. Suddenly, he's gone, a red and blue streak speeding towards a back alley somewhere as he catches a bullet he saw being fired by a common thug during a mugging gone bad. He quickly subdues the bad guy. However, while doing so, Superman hears the sound of a small car impacting a tree in the opposite end of the city. He can hear that the driver's heart has stopped. The accident was fatal. Superman COULD have saved that man, but then he wouldn't have been able to stop the murder in progress here. Superman made a choice. Someone lived, someone died. Superman makes that choice several times each day. And as Clark Kent, he sometimes has to choose to save neither.
That's Superman. He can't save everyone, but he knows they're suffering and dying... because he chose to be elsewhere. If you can turn THAT into a game. Making the hard choices, living with deciding who lives and dies, on a daily basis. Then you have made a Superman game.
And that also means that his powers, as fantastic as they are, don't really matter that much. If Superman chooses to do something, he can, except being everywhere at the same time. It would be a fascinating game, but probably not one that sells a lot. But imagine having to choose over and over and over. What do you choose? What is Superman to YOU? A diehard crimefighter? A defender of the weak? A guardian angel? Or a mild-mannered reporter at the Daily Planet?
Because being Clark Kent is also a choice Superman makes. It keeps him grounded. But while he's playing human, he is deliberately ignoring people in need. And he has to. It's his time off from the sensory overload of being able to see and hear every little accident, crime, or other emergency that Superman could solve.
So yeah. An open world Superman game done with an eye for this would be immensely fascinating. But I doubt it'd be that fun. Then again, being Superman? Probably isn't that fun either.