They're an automatic outcome of fairly benign starting conditions in GR: Simply having more than a certain amount of mass within a certain volume.
The required density for that stuff goes down as the volume goes up, so a solar-system sized object (~77 AU radius) at mere normal sea-level (Earth) air density (1.2 kg/m^3) would just be one automatically.
Given that the maths of GR requires spacetime to not have a singularity*, and yet it predicts a singularity from benign starting conditions, I take this as a sign that GR is not correct.
But black holes are nonetheless an outcome, not a presumption.
