Einstein's theory of relativity is based on a mathematical derivation and a single experimental observation which demonstrated that the speed of light is constant regardless of the position or speed of the measuring device. The pure mathematical consequences gave rise to the special and general theories of relativity. All the experimental evidence collected since have not been used to expand or correct the theory, but have just shown it to be absolutely correct, right down to the most extreme accuracy that scientific instruments can detect.

Mathematics is inescapable. If you want to understand why the speed of light is constant, I don't know the answer to that, probably no one knows, or ever will.