I was thinking the same thing. It is totally possible to link to our contacts so why haven't they
Why should they? It's like asking Apple to allow Android to use iTunes. Google wants you to manage your contacts on gmail. Much like Apple wants you to manage things in its own ecosystem.
It's a matter of who's ecosystem is more worthwhile to the consumer. Apple's long standing strategy is to create and grow ecosystems but at the same time lock you into it, for better or worse.