It is not about Google is keeping revenues but Google itself is receiving lesser revenue from advertisers. There are many factors behind loss of revenue.
-- Competition is going tougher, there are many website offering low cost advertizing, so Google also has to cut its rates to attract advertisers. Before there was no real competent of Google Advertizing.
-- 2-3 years back users used to click on Google Ads thinking that ads are part or offer by websites but now everyone aware that those Ads are from Google and outer advertisers that they already seen somewhere. So less clicks means lesser revenue. Two years back I used to get 10 click of 100 views but now days getting 8-10 click per 1000 views. Its awareness.