black holes do not emit appreciable amounts of radiation themselves. Their accretion discs do. That in turn requires mass influx large enough to form an accretion disk with enough friction to radiate. The gas in the galactic halo and for that matter even the interstellar gas in the center of the galaxy[0] is just too thin on average to provide that. We only see them when they're draining mass from a binary companion or swallowing a gas cloud.

[0] https://www.sciencenews.org/article/dozen-new-black-holes-fo...