I guess it's part of Googles Ads's endless battle against "robot" clicks. A site as big as SO should not use Google ads, but instead use their own ad service. Just make an automated system where people can signup and show an ad. Make it cost 1$ per 100 page views. That would probably earn SO two orders of magnitude more then they get from Google Ads.
As an advertiser, yes, but on Google Ads you know that 90% of those will be fake ¹. And as a publisher on Google Ads you only get something like 1$ per 10000 impression ². Advertising directly on SO you know all views are not only legit, but also target at developers, so I think advertisers are willing to pay more. While most advertisers are paying per click, the whales only care about impressions, not clicks (TV commercials).
1) Measured by analyzing the traffic I got from Google Ads
2) That's what I get from Google ads as a publisher, but you used to get a lot more in the epoch, like $5-10 CPM
