Because it’s an inertial frame, to the accuracy required, but the Earth’s frame isn’t.
Mind you, I can’t claim to know anything about GR, certainly not by this standard. My expectation is that when one does the mathematics, GR does not predict planets spiralling to destruction on thousand-year timescales, and that shminux has located the problematic step. But I do not know the mathematics. My expectation is also that verbal stories to justify why it does or does not happen are only helpful once one has worked out the real mathematical story, and cannot by themselves resolve the matter.
Because it’s an inertial frame, to the accuracy required, but the Earth’s frame isn’t.
Mind you, I can’t claim to know anything about GR, certainly not by this standard. My expectation is that when one does the mathematics, GR does not predict planets spiralling to destruction on thousand-year timescales, and that shminux has located the problematic step. But I do not know the mathematics. My expectation is also that verbal stories to justify why it does or does not happen are only helpful once one has worked out the real mathematical story, and cannot by themselves resolve the matter.