Is it because they eat more than they eject? Or something else?
Edit ------- I mean black holes which have just started forming from stellar collapse. Should had mentioned this.
Edit 2 ----- When you have a ball of neutrons which is compressed to the maximum extent physically possible during stellar collapse, the spatial curvature would be maximum at the center and would decrease as we travel away from it. However even at this point nowhere is the curvature enough to prevent light escaping. Now somewhere between this point to the formation of singularity I suppose the 'light trapping' curvature would develop at the center and then expand outwards. So that should mean that the event horizon grows from zero to its final size in a non-zero amount of time. Even inside a black hole as far as I know the curvature decreases outwards. By this logic the Hawking radiation should start off as soon as the EH develops and should counter if not stop the growth of the black hole.