Because Gedo Mazo is human sized?amirite?
Any other nin is getting warped and ends up rotting in Kamui land no diff.
Did you think this through at all? Let me make it simple for you. While standing 50 feet away, throw a stone at the empire state building. Aim for a window; My bet is that you'll miss that window but you'll at least hit some part of that non-moving building. Now, again standing 50 feet away, throw a stone at someone's head. If you miss by even a little, you're completely missing the target.
What's the point of all this? Well, for a large object, a small offset still allows you to hit, at least, some part of that object. For a small object, a small offset would more than likely result in you completely missing. If the kyuubi tried to sidestep kamui, it'd probably have it's arm ripped off. If guy did it, kamui would hit some random stone and kakashi would have just solo'd himself from exhaution.
Anyway, I won't go any further into this because people already believe kamui is instant even with all the evidence to the contrary... In fact, the manga has pretty much stated that kamui and amaterasu work the same way...