Rigidity is a major factor--if you inspect one you'll notice that high speed and high repeatability machines (e.g. a 6-axis +/- 0.020mm repeatability manipulator) are made from rigid (often high-mass) components with robust joints.
I see. I wonder if my hypothesis is still valid, even given lower quality rigidity. Human arms aren't made of big heavy metal rods, yet we can still draw beautifully. Couldn't cameras and software compensate for cheap unstable draw arms?