People perceive high-density products as high-quality, and low-density products as cheap plastic crap. Numerous products have included weights for this reason... take a look inside your mouse. People want their phones and laptops to be light so they don't have a brick in their pocket or backpack. Light * high density = low volume. They don't want to reduce the screen size, and bezels are already minimized, so the only option to reduce volume is to make it thin. Of course, once they make it thinner the advertising department will hype that feature, but the real driver is density.