Google seems to only have gotten the address coverage it has by sending vans around with cameras and then making people type in the addresses for CAPTCHAs

There is also a lot of data that has been collected by governments:

http://results.openaddresses.io/

There's a variety of reasons that such data hasn't made it to OSM (resources, licensing issues, disagreement about how it should be integrated, disagreement about whether it should be integrated, etc.)

Google is also a household name, so people and companies ensure they they enter their business into Google Maps.