I think the unstated (but highly prevalent) view among executives in large swathes of this industry is that they don't really care to spend any time or money on user testing or quality assurance, and if this role exists at companies it is usually under-compensated and straddles both these functions to have some party be accountable. It is sometimes a check on product teams and vision-driven executive teams who don't prototype/test their ideas (or empower their teams to do so), and sometimes a check on engineers and engineering managers who don't want to be accountable to gaps in quality.