It’s because the universe expanded (faster than c) and those photons got a free ride.
How can the universe expand faster than c? Isn’t it the barrier that can’t be broken? I hear the trendy physicists on History Channel theorize that the universe expanded at faster than c - in the moments after The Big Bang. Yet, they don’t even give passing notice to Einstein’s Theory of Relativity.
If one carries this through to its logical conclusion, what we are seeing is 45 billion years old FROM OUR PROSPECTIVE, yet it is only 15 billion years old, FROM ITS PROSPECTIVE. Something ain’t right.