Source: Apple Insider
quote: It's not always practical, or possible, to change DPI. Usually, you can only optimize text, but window elements (panels, buttons, scroll bars, etc...) don't always change.
quote: What you said only makes sense if the number of pixels in window elements scale with changes in resolution. For example, a taskbar that goes from 72 pixels in height at 1280x720 to 108 pixels in height at 1920x1080. The taskbar would take up 10% of the screen in both cases, but a JPG would take up less screen percentage on 1920x1080.