They don't. I spent around $2M from 2003 to 2015 and clicks became steadily more expensive and useless. The advice from their "experts" usually resulted in more spending with lower ROI. Eventually we stopped using Adwords. Google obviously didn't even notice or they just didn't give a shit, as they never contacted us to ask why we stopped using AdWords.
You cannot imagine how great AdWords was in 2004. It was much more transparent and easier to use, and the ROI was just incredible. Their success at that time was more than justified. Not so sure about today, though.
It could very well be that real users are disregarding ads (either with technical solutions such as blockers or by just ignoring them) and fraud has picked up the slack so that overall the number of clicks hasn't changed but the ratio of real user to fraudulent user has changed significantly.
You cannot imagine how great AdWords was in 2004. It was much more transparent and easier to use, and the ROI was just incredible. Their success at that time was more than justified. Not so sure about today, though.