Blame YouTubers/Influencers for that, they constantly push for uniformity with "comparisons".
Samsung released a new phone?
Top phone YouTubers will quickly compare how they like the feel of iPhone or Pixel vs Samsung and how it holds on their hand, how it's matte, how it's sized and companies want to win those points. So satisfying them is a priority.
So we end up with matte black, thin, rectangles.