As of today, some ads pay per click (CPC), but most ad spots pay per 1000 views (CPM). Ads can influence behavior after they are viewed, regardless of whether the user decides to interact with the ad. I'm sure Google has put tons of effort into trying to tie ad views to purchases, both online and offline (I bet GMail and Google Pay are leveraged for this).
I am not claiming this is good or bad, but clicks are not a good enough signal of efficacy for the vast majority of ads shown on the internet.
A good ad network's JS should be able to tell how long the ad was in the viewing portal, or for a video interstitial how long it played before the viewer skipped the end. That sort of signal could be useful, especially the way many sites use horizontal ad banners like horizontal rules in the pages.
They would never implement it because there is no valid signal here. The vast majority of users would just thumbs down every ad they see because they believe that will result in less ads.
Go check out the messaging around Ad Choices and how poorly it ended up working.
Ad networks are in business based on ad performance, which is driven by the user. Even though there's quite a lot of terrible UX with online ads (for lots of reasons), the user does matter more than you think.
But.. Ad networks will never implement this, cause priority there :
1) ad network 2) advertiser 3) publisher 4) user
This bumps user from 4th place to 1st place