The nature photographer John Shaw used Kodachrome exclusively at one point in his career, and if you look at his books produced during that period the photographs are very sharp. He demanded high quality reproduction of the images in his books, for obvious reasons. Magazines will use a variety of scanning and printing processes, with variable results. Something else that often occurs in editorial situations is that art directors will demand using images that are sometimes extreme crops of photos, and they often will look terrible to someone who routinely is concerned with resolution and sharpness in images. But, I suspect your publication simply does not have a good Kodachrome scanning workflow established, which is a pity.
By the way, I agree that the National Geographic magazines and books have been very powerful inspirations for generations of photographers, but if you pull them off the shelves and look at ones produced in the 1960s and 1970s, the photos often look really quite fuzzy, even though you know that the cameras, lenses and films of the time were perfectly capable of producing sharp images. It was the contemporary printing methods; the mass production, economical methods of those decades were simply not able to reach the "high definition" standards that are so common today, and people did not expect it.