I don't think it was just a main plot point in this movie. I imagine that it will be a key, ongoing theme throughout this franchise. That's pretty much modern Lex Luthor's bread and butter. So even though we didn't see a wide view of humanity's reactions, I suspect we're going to.
Here's the thing...while the idea of Superman and the Kryptonians being Earth's first contact with aliens was part of the movie, it wasn't the central storyline itself, but more of a subplot. The central storyline was in fact, Superman's origins as a heroic figure. And we did see how humanity dealth with alien contact in several respects, in the context of how they dealt with Superman, and in the context of Lois Lane's story, Perry's attempts to keep it under wraps, etc, and the sensationalization of it all when it went public.
As far as world leaders sequences, etc, let's be honest, we've seen those scenes, in countless other alien invasion films. There's nothing new there, and nothing truly unique about that anymore, certainly nothing that necessarily serves the Superman mythos. What we haven't seen quite as much of is an alien sitting down with military leaders and offering to help. The dynamic in this movie was different than your typical alien invasion/disaster film, and for good reason.
This was a superhero movie. It was a movie about Superman, and how people would specifically react to Superman and his kind. In the context of how the world would react to hostile aliens, which is what happened, we saw what we needed to in order to maintain focus in a story with a central character like Superman: The military's initial and ongoing response. Since it was pretty much only on American soil, we only saw the American military.
The filmmakers have a responsibility to focus on scenes and ideas involving Superman and the core concepts of his mythos, not tick off boxes of things we need to see in every alien invasion film.