Comment by fnordpiglet

Well because in relativity accelerating frames of reference are pretty complex, space isn’t flat, neither is time, and the position of something is a fairly complex concept as is causality etc. Momentum changes depending on what frame of reference you use, and space itself can’t be a frame of reference. That’s because momentum isn’t actually a thing but a measure of things within a frame of reference with respect to each other. Energy, mass, etc these are things - but momentum itself isn’t other than as a way to describe the needed energy to align the frame of reference into a different configuration of relative motion, mass, and energy. Its conservational nature is derived from the fact those things are conserved.

Now of course Newton and most people before and after him through to Einstein did just fine ignoring all these things and just “picking something” worked for everything they needed to do. But it was at its most fundamental nature wrongs and position is not simple. In fact due to relativity even ignoring quantum, you can’t even know your relative position with any certainty - just your relative position as of some distance in time proportional to the relative distance you observe.