Because it is all around us, all the time. Every cubic centimeter of space in our universe has about 400 photons from the Big Bang. These photons are, of course, constantly moving through space in random directions, but in any one instant, every cubic centimeter of space has about 400 of them. The Big Bang, you see, did not start 'out there' and eject us to where we are now, but instead it was a process that happened in every cubic centimeter of space. Of course now, thanks to expansion, there are a lot more cubic centimeters of space in our universe than there used to be. The stretching of space has also caused the temperature of the fireball radiation to get shifted downward from its original billions of degrees after the first second of the Bang, to 2.7 degrees after about 15 billion years. By the way, space has in fact expanded faster than light through much of the early history of the universe, this has made it particularly difficult for the fireball radiation to propagate from place to place.