On the iPad, the thick bezel area surrounding the display serves an essential purpose — it gives you a place to rest your thumbs while holding the device. The non-display “forehead” and “chin” on the front face of the iPhone serve no such practical purpose. All Apple needs is enough room for a home button at the bottom, and the speaker, camera, and proximity sensor at the top.
No. The width, in landscape, of the forehead and chin of the iPhone is roughly the thickness of an average thumb. And both have identical widths; they are symmetrical. My guess as to why there are such thick top and bottom bezels with roughly the thickness of an average thumb is: So you can grip the iPhone with your thumb, left or right.
Think about how you hold the iPhone when taking a photo. I find my thumb needing that forehead or chin to hold on to. That thick bezel also comes in handy when I’m playing games, especially games where you need to tilt the iPhone to turn left, right, go fast, or slow down. The forehead and chin of the iPhone serve important practical purposes, and will not go away.