This GIST provides an example of a Google Maps map with an OL3 map as control, to give users a Google base map with OL3 content on top.
Open the corresponding bl.ock to view the example in your browser.
This integration between Google Maps and OL3 is rough. Do not use it in production!
This question very often comes up on the OL3 mailing list. The short answer is because Google doesn't allow us to directly access their tiles. See Paul Spencer's post on the OL3 mailing list for a more complete explanation.