Let's totally disregard games for a moment:
This may be self perpetuating, but as I understand it the Mac platform doesn't really have much 3D graphics intensive workstation software available. Users of Photoshop tend to overestimate their own need of graphics hardware, but apart from games, what need do OS X users have of 3D performance if they do not dual boot into Windows (or even Linux - I have to admit to a serious lack of knowledge about the state of Linux based platforms as 3D workstations)?
This is a serious question - I haven't really been part of the field since the battle stood between Alias|Wavefront and Softimage|3D. (I have to admit to liking LightWave 3D on the Amiga though, even though I always loved SGI hardware).