That question is so basic that it has no answer. All physics can tell us is that if we assume that it is true, we can solve many problems and predict the results of many experiments. If we assume it is not true, we get the wrong answers.
Why does gravity exist? That is another basic question. Why does the world exist? The same.
In science we try to reduce our "why" questions that we cannot answer to a minimum, and those few we call "Laws of Nature". We use these Laws to explain all the other questions that there are. They seem to be true because they work! We may learn in the future that they are not quite true, but until then, we accept them.
Sorry that this sounds so mysterious, but it is true. It is part of the philosophy of science.
E-Store
why is the speed 'c' ultimate?
why we can't move an object faster than c or at c?
