Google glass is able to get away with being so slim and small because it does far less than something like Microsoft Hololens. It's the same with VR headsets like Oculus Rift and HTC Vive.
It's technically less demanding to make an entire complete world virtually than it is to integrate a virtual world with a physical one. Of course, that's all pretty obvious, but I wanted to point it out to any that might walk away thinking: 'Hololens sucks'.
It's incredibly impressive from a technical standpoint, just not quite commercially viable yet. When it does get there (probably in less than 5 years), it will become a billion dollar industry.
