Because a) google is better at indexing a given source, and b) every user shouldn't have to build their own search aggregator across the n sites they have privileged access to; that's the role of a search engine (except for it covering all sources not just a few).
Maybe there's an opportunity for Google to partner with subscription sites to index their content for users who own a subscription -- but what should Google do when there's a ~0% chance a site contains useful information for a non-subscriber?
