We don’t know.
What we do know is that what’s usually called “the speed of light”, is better thought of as the maximum speed at which information can travel. Light can carry information, so it’s capped at that speed. So is gravity (if the sun disappeared, the earth’s orbit wouldn’t change immediately, it would take time for the change in gravity to reach us). But that just moves the question–why would the speed of information be what it is, and not some other speed?
There’s a Nobel prize for you if you figure it out.
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