But then it's responsive to the page size, not the container size. That's a small, but important difference.

For instance, I recently made a component to be included on other pages at work. It had to look good no matter the size the page gave it. So they might put it into a 400px wide container, on a 1600px wide page. If I used vw or media queries to draw it based on page size, it would behave wrong.