Why not? Mac pros are expandable.
Seven graphics cards. That sure is expandable.
Let's leave the Mac Pro and MacBook Air out of this next bit: You can replace everything in every Mac except the processor and GPU.
With the Mac Pro, you gain the ability to change processors (to something in the same line, either faster or slower) and graphics cards (to one of the other six cards). You can also change the CPU in the iMac and Mac Mini.
You want expandability, but OS X doesn't have the drivers for it. Why doesn't OS X have the drivers for it? Talk to ATI and nVidia.