Ask HN: Why srcset and not just progressive JPGs?

These days I often use devices with high DPIs. Some auto-scale websites up, on other devices I use tools like NoSquint that sets a default zoom. So more and more often I wish the images on my sites had a higher resolution.

Looking into the documentation of srcset, I wonder why all the complexity. Couldn't we just use progressive JPGs and the browser loads until it has all the pixels it needs for the image in the current situation (screen dpi, zoom level, css scaling etc).