Ads are certainly part of it, Google often places three large ads above results. It's probably also partly due to the maps, places, shopping offers etc that are often placed ahead of real results too.
While financially rewarding to Google they should be alarmed that only two thirds of their users actually find and visit a website. They ARE a search engine despite everything else they try to be.
I would tattoo 'Google' on my shoulder if they would get rid of places, shopping and maps from serps. It will never happen.