You have one of the major questions that a lot of people have about science. WHY? Unfortunately there is no good answer. A lot of times it is claimed that science is the quest for why things are the way they are. That is not strictly true. Science cannot answer the question "Why is it this way?". What science can do is to describe how the universe works. Let's take a quick example of Newton's law of gravity. His answer to why the planets move the way they do is that gravity directs their motion. But when it came right down to it he couldn't say what gravity was, it is just a force that works according to his law. Then Einstein came along and extended his work with GR. So now gravity is the result of warped spacetime. But we still don't know WHY spacetime is warped.

You are wondering what force caused spacetime to warp. But there is no force causing the warp. The warp just is. When we get right down to it, at the most fundamental level science can only say "That's the way it is just because that's the way it is". You are wondering about what warps spacetime. Well, if you think about the electromagnetic force there is no explanation of why the electric charge exists. Another case where science can understand the interactions between particles, but why the interactions exist is beyond the capabilities of science.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.