There are lots of plausible hypotheses, that don’t contradict existing known data, for why we don’t see evidence of advanced ETs.
Why are we spending so long discussing a hypothesis (black hole clusters) that is nowhere near attaining that status?
Agreed.
And as mentioned upthread, and ignored, we have performed gravitational microlensing surveys.
To the extent that our galaxy could be littered with black holes, their size and/or distribution seem insufficient to affect most lines of sight across the entire galaxy.