It's a very large horizon, so the Hawking radiation is very weak (it's always a weak effect). We can't see it because the cosmic microwave drowns it out. Broad

It's a very large horizon, so the Hawking radiation is very weak (it's always a weak effect). We can't see it because the cosmic microwave drowns it out. Broadly Hawking radiation intensity goes with curvature - i.e. a more curved surface has a better chance of separating a particle pair then a flatter one. This is because a sharper curve means more vectors which carry you away from the event horizon. This is also why small black holes evaporate faster then big ones - as the black hole shrinks, the Hawking radiation intensity increases because it's curving more and more (hence why microscopic black holes don't devour everything).