HP can control the full stack. MB and PSU at the same time. But smaller OEMs might want to not have to manufacture their own totally custom components. So it ma

HP can control the full stack. MB and PSU at the same time. But smaller OEMs might want to not have to manufacture their own totally custom components. So it makes sense for them to standardize. If not for interchangeablity then to ensure you can sell your custom parts to other OEMs.