A guess: The Big Bang resulted in lots of <i>pieces</i> some of which were black holes, of a wide variety of masses. In particular, the black holes of billions of solar masses weren't <i>formed</i> from collapsing stars and didn't <i>grow</i> to that size by <i>accumulation</i> but were <i>born</i> that size. And for <i>dark matter</i>, that's from many black holes of tiny masses.