Well, there's also the fact that some "UX" guys don't have any great ability in graphic design, but are great when designing the use and flow of an interface. It's not necessary that an ability in the latter implies similar ability in the former.
Which "UX guys" are these you’re thinking of, who don't have a competent understanding of graphic design? I posit that it is irresponsible to claim to be a "UX guy" and not try to learn quite a bit about spatial hierarchies, data presentation, human response to colors and visual relationships, etc.
Though graphic design surely includes all those things you've mentioned, I was using it in the sense that most "web designers" might use it, as in the ability to make pretty stuff in photoshop. I'm not sure that someone who can, for instance apply Tufte's advice on data representation, necessarily also makes nice looking logos.
Equating “don't have any great ability in graphic design” with “can’t draw logos” is applying a pretty narrow definition. Someone who just “makes pretty stuff in photoshop” should probably be called a “digital illustrator” or a “photo retoucher”, or a “digital imaging expert”, not a “graphic designer”.