All massless particles move at c in a vacuum, relative to any observer. Were this not so, contradictions with the fundamental postulates of relativity would appear. Maybe those postulates are wong, but they haven't let us down yet. Also note that it isn't true that photons move at c in their own frame, anything is at rest in its own frame if it has a frame, which massless particles do not (no observations can be made from the frame of a photon, so it is not an allowable frame for doing physics). Finally, the question of why c is what it is is very profound, and I have no idea the answer, except to say that there are basically three possibilities. Either c=infinite, which would do violence with the principle of cause and effect (causes and effects could happen at the same time), or c=0, which would not allow anything to cause anything else, or c is somewhere in between, which allows for an interesting universe that acts like ours. The value of "somewhere in between" seems pretty arbitrary, but it is important that it is a very fast speed relative to our daily experience. Were this not so, even cavemen would have attained some level of understanding of relativity. Why is it such a fast speed relative to our experience? I've no idea.