Adblocking is very much on googles radar. But they realise it is a cat and mouse game - and whenever you start playing that game, you run the risk of ending up in a position worse than you started with. They currently get 80-90% of the ad impressions they try to display, which is pretty good compared to a hypothetical future where someone like Microsoft makes an adblocking-by-default browser and courts force Google not to block them.