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How?
Gravity at the event horizon is inversely related to the mass of the black hole. So for a supermassive black hole, gravity at the event horizon can be weak. But you still can't escape because it's too large.
Imagine light trying to escape the Earth's gravity. Its path is slightly deviated by the Earth, then it gets far enough away that the Earth has little further effect.
Now suppose at that distance, it still experienced the same gravity. So the trajectory of light is deviated a little more. It keeps moving farther away but gravity barely changes, even at huge distances. Eventually all those little deviations add up and it's going back where it came from. Light can't escape. It's a black hole.