Einstein's theory of general relativity describes gravity as the warping of spacetime, but it does not explain why mass or energy causes this warping. The gravitational effects attributed to mass-energy are known through empirical observation rather than mathematical deduction. The discussion highlights that the source of gravity is the stress-energy tensor, which includes mass, energy, momentum, pressure, and stress, indicating that gravity is not solely caused by mass. The conversation also touches on the philosophical aspect of asking "why," suggesting that some questions may ultimately be unanswerable and do not contribute to scientific knowledge. The limits of scientific inquiry are acknowledged, emphasizing that some fundamental questions may remain unresolved.
Also don't forget that curved spacetime is just a mathematical model. You could just as well ask: Why does force change the velocity of a body. The answer is: Because force is a mathematical abstraction that was defined this way.
