Because integrating the document into a page is annoying - in order to fit other content such as ads, the main viewing window is restricted to an unreasonable size. I'd much rather have it open up a document in the full screen.

Plus there's a certain senselessness to distributing documents this way when everyone is already happy using a pefectly capable, open file format that every machine can read as it is, not to mention tying the existence of said documents down to the survival of a single company.