Imagine you are Valve or ID or Dice, and your team is going to create a new engine to run your company's main titles for the next decade. You want an engine that is innovative and flexible, can knock socks off next year and still impress gamers 5 years down the road.
Would someone in this position use helper libraries like GLUT aor GLFW or GLM or would they create their own libraries for their project and do the win API stuff manually?