Because there's a lot you can do with a browser these days. You can even run offline apps and native code (Emscripten, NaCl, PNaCl).

Also, if you turn on developer mode, you can access all the Gentoo bits that hide underneath the Chrome bits.

Consider common use cases - word processing, social media, organising photos, communication - all can be done with a stock Chromebook. Once developers realise the full potential, you'll see bigger, better games that run on Chrome (check out Bastion and From Dust on the Chrome web store - pretty impressive already).