You haven't really made an argument that establishes why they can't just adapt some of the more classic Superman stories. And if the argument is that Superman stories are too far out there or fantastical, well, I'd say that the MCU shows that audiences are
more than willing to accept out-there and fantastical stories. I mean, if they can basically adapt Planet Hulk in all its outlandishness and the Infinity Gauntlet storyline, then I find it hard to believe that something like "What's So Funny About Truth Justice and the American Way" or "For the Man Who Has Everything" is too out there for moviegoers.
And on top of that, famous Superman stories have already been adapted. Again, Death of Superman basically inspired BvS and was also adapted in the Justice League animated series. For the Man Who Has Everything was also adapted in Justice League Unlimited.