Why do browsers not render pictures in native pixel density on retina display?
This is intentional. Most websites aren't designed for retina displays and only have 1x assets. If these 1x assets were drawn at native resolution on a retina display, they would appear at one quarter the size (half the height, half the width). That would obviously be impractical: it would be like browsing the web with your browser's zoom level set to 25%.

To get around this, the browser simply draws everything at 2x (retina) by default and then upscales any 1x assets to match. This way, websites appear at the same physical size on a retina display as they do on a non-retina display.