Because the graphical difference between max settings and medium / high settings is often not that much. Try it yourself.
Depending on the game there can be a noticeable difference between medium and ultra settings as a result of additional fancy effects such as shadows, lighting, ambient occlusion etc that really serve to create realism. I am a veteran PC gamer since the early 90's, there isn't much I haven't seen the difference between over the years and I consider my opinions well formed based on that experience.
Anyway, we have both said our piece so I am happy to agree to disagree at this point instead of getting into circular arguments.