Making an LCD have a high ppi is not difficult. You had lcds with greater ppi's 10 years ago for LCD projectors and what not. A unibody laptop isn't breaking ground from a manufacturing stand point, it's just specifically shaped piece of aluminum.
Why did the iPhone run at such a low resolution while Windows Mobile and Android phones were coming out with 3x more pixels? Yet when they increase the DPI game slightly, brand it as a Retina Display, you think they're innovative?
As far as the aluminium goes Apple has fetishised their materials choices for a long time and had as many hits as misses e.g. "Titanium" paint that flakes off, White iBooks that discolored, white iPhone 4s that they can't manufacture. Others generally don't bother as their customers don't want or need it. I'm not sure Apple users do before they are trained to by Apple.
Metallic laptops tend to have wireless reception issues, conduct heat a bit too well onto consumer laps, have warping issues and can add weight. All of these problems have been present in macbook pros.