Because they are both massless. Everything massless moves right along the "edge" of these gaps/shortest paths in spacetime. Since it's the shortest path through spacetime there's no quicker way to move than that. Humans can imagine shorter paths, i.e. FTL ones, because we don't correctly intuit the actual shape of the universe. It's like how there is no shorter way to move along the surface of the Earth than going along a great circle.
Oh cool, it suddenly clicks into place! No guarantee, I will get it an hour from now, though.
Thank you so much for taking the time to explain all this to us, it means a lot!!
What if Eleanor Roosevelt had wings? -- Monty Python
One important characteristic of a theory is that is has survived repeated attempts to falsify it. Contrary to your understanding, all available evidence confirms it. --Subbie
If evolution is shown to be false, it will be at the hands of things that are true, not made up. --percy
The reason that we have the scientific method is because common sense isn't reliable. -- Taq