Let’s go back to the infancy of the universe, right after the Big Bang. You have a lot of primordial gas and dust that’s just been flung around. A lot of it. Some of this aggregates into globs so massive, they collapse into ultra-dense bodies that basically seed black holes on the spot. They begin feasting on the fresh gas and dust of the infant universe, without any sort of pressure valve to switch off the feeding frenzy. Eventually, the radiation produced by all these black holes and by other newborn stars in the universe puts an end to their massive proliferation and growth, leaving what we now call supermassive black holes.