The folks at iFixit write:
The Retina display is an engineering marvel. Its LCD is essentially the entire display assembly. Rather than sandwich an LCD panel between a back case and a piece of glass in front, Apple used the aluminum case itself as the frame for the LCD panel and used the LCD as the front glass. They’ve managed to pack five times as many pixels as the last model in a display that’s actually a fraction of a millimeter thinner. And since there’s no front glass, glare is much less of an issue.