What if spacetime curvature and mass are coincidental, circumstantial, and are not cause and effect. Or what if spacetime curvature causes mass? There was a comment quite a while back pertaining to the old vortex theory of gravity, that if a planet were not there, say, Jupiter, its gravitational field would still be present, that the presence of gravity causes the planet to form right there. This is just to point out that these are all hypotheses and that flat statements might close some doors to speculation.
Nor can I prove that Invisible Star Goats are NOT pushing the planets around the Sun, instead of reacting to the influence of the spacetime curvature (gravitation).
The point is that General Relativity is a Theory that fits all available evidence and which has survived all attempts at falsification. "Vortex" theories of gravitation notwithstanding, no other gravitational theory can make that claim.