Because Einstein is assuming that the speed of light is constant, so what he describes or what he wants to describe may be when the speed of light is constant.
But if the speed of light is not constant, then, we cannot use Einstein's idea. His ideas may not be relevant if the speed of light changes.
If the speed of light is not constant, then, it is no longer an absolute entity that determines the universe we live in.
Another entity must be there to determine how light flows, and this is time.
To me, there must be an absolute time that determines how the speed of light changes, and there are also individual times, where individual time and absolute time depend on one another.
But my writings are different to Newton, because to me, individual times can change. Individual time can flow back to the past or to any other direction, but the universal absolute time can only flow forward.
And it is this absolute universal time that is flowing forward that determines how the speed of light changes. It is also this universal constant time that shows how the individual time can travel backward, moving back to the past, or to any other direction.
So individual time can flow to the past or to any other direction, while the absolute time is constant, flowing forward and the constant universal time keeps a record of how individual time changes its speed and direction of flow.