Because gravity waves, and the gravitational field are not the same things.
Gravity waves are propagating, time-dependent changes in the local gravitational field that travel at the speed of light 'in' space-time. Inside a black hole, they cannot get outside the horizon. The gravitational field, which is the static distortion in space-time upon which the gravitational waves are seen to move, remains unchanged.
The gravitational field near a black hole does behave in a very non-intuitive way, but its behavior is completely accounted for even in Einstein's 'classical' relativity theory. The horizon will not collapse as seen by an outside observer, because the matter that collapsed to form the black hole is actually still OUTSIDE the event horizon as seen by the external observer. Because of the enormous time dilation factors that occur within a few microns of the mathematical horizon, it will seem as though the entire star's mass is concentrated just outside the horizon, and it is this mass which produces the external gravitational field seen by the external observer 'at infinity'. If you were on the surface of the star, however, you would see a very different reality and within a second you would fall past the horizon with the surface of the star, and be crushed into the central singularity.
Copyright 1997 Dr. Sten Odenwald
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