Mass bends spacetime. Black holes are so incredibly massive that they bend spacetime around them so incredibly much, every single direction points to the center of the black hole.
In other words, if you cross the event horizon of a black hole, every direction you travel in thereafter would actually be towards the center. This is true for light as well.
Basically, once you cross the event-horizon, space turns inside-out and the center of the black hole surrounds you entirely.
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