Because there is no way to determine that motion, without reference points. Because that motion does not exist. Nathan is now taunting Ken - the difference between acceleration and motion being cited here. And, when dealing with time in a material system, that's a big difference - let me tell you. As I said before your logic is non-existing.
Motion is a part of a material system when there is a medium occupying space.
Ken is somehow able to look past the acceleration problem. I think Ken may score here. This is the only way to conduct science, if your theory doesn't have any observable consequences then it's metaphysics - and should be treated as such. Maybe the match will be maneuvered to the 'Metaphysical Mosh Pit'. The situation is similar in General Relativity.