system
Given the following post, respond as if you're commenting on Reddit:
human
Why does light travel the same speed for all observers?

To my understanding, if something (say a ship in a vacuum) is travelling away from a light source it would observe photons reaching the ship in [time=d/c], while a stationary observer (with respect to the light source) could theoretically observe photons reaching the ship in [time=d/(c-v)] (where d=initial displacement from ship, c=speed of light, v=velocity of ship).

I use this concept whenever I want to remember the implications of time dilation for something travelling near the speed of light, but I realized that I do not understand why this phenomenon should occur at all.  My limited search on the subject found only that this is a mystery of nature and has been proven by experiment.  Does any have some insight on this matter?
gpt
I wouldn't classify it as a "mystery" of nature. But it is a postulate, something assumed to be true, based off of a lot of observations. 

The postulate arose because people were playing around with the equations that govern electricity and magnetism. You can test each of the 4 equations independently, they're called Maxwell's equations. And when you put them together in the right way, you get a wave that has some velocity that's equal to 1 divided by the square root of the electric permittivity constant multiplied by the magnetic permeability constant. But as you look at the equations, no where in there did it mention the speed of some observer. Each observer would necessarily come to the exact same conclusions based on the same laws of Electricity and Magnetism. 

So Einstein ultimately becomes famous for saying "fine, let's just make this a thing. This speed of electromagnetic waves is a constant value for all observers. And no physical experiment can distinguish between a frame at rest and a frame in uniform motion." Take those two postulates and you get the whole of special relativity.