Likely because, in space, reflective items don't heat up as much from sunlight as would a dark, energy absorbing, material. Shedding heat in space is a difficult enough endeavor already without also painting your satellite black so it reflects less light (and thereby absorbs more heat).
They aren’t in the Earth’s shadow at the start of the night
> For the SpaceX satellite mega-constellation, he found that dozens of trails would appear in each image taken two hours into the night with ESO’s Very Large Telescope (VLT) at Paranal Observatory in Chile
Not to mention the satellites of Reflect Orbital whose sole purpose is reflecting sun light into night areas