To my best knowledge...
1. You can't make a single cent if your bot visited facebook.com 100 million times. You can make some serious money if your bot visited some-exciting-domain.com, which belongs to you, and there was 5 ads displayed on each visit.
With this incentive you have all the reasons to make your bot very human-like (think headless chrome, realistic mouse movements, having old cookies, etc) so fighting fraud gets extremely hard.
It's easier to serve the ads and let advertiser figure out anti-fraud measures by himself. Being responsible for measurements and lack of fraud on open web is a huge PITA without clear path to huge uplift in revenue.
2. Facebook optimizes UX (or claims to), and calls to other servers make site slower, especially on mobile, lowering user engagement. This argument obviously does not work for some-exciting-domain.com. So, you can call whatever your want from your ad on some-exciting-domain.com, but on facebook.com you play by facebook rules.
In fact, some-exciting-domain.com can probably ban ads which call other domains, but it will just kill his revenue (programmatic systems will label him as "non-performing", because nothing is properly measured and stop buying ads there).
Is water still polluted if more and more and more are willing to chug it?
I don't have a link on hand, but GOOG and FB captured something like 95% of digital advertisement growth in 2017. In other words, out of each new 1$ shifted to digital from TV and print, 95 cents went to duopoly.
And advertisers which are still "chugging" open web, installing more and more "filters" and "purifiers" (different anti-fraud and measurement providers).
People from digital media are talking about digital media crash. [0] Buzzfeed failed their revenue goal and fired 100 employees. [1] Mashable was sold for peanuts. [2] Business Insider, granddady of ad-monetized clickbait, pushing more and more articles under "BI Prime", which means paid access.
A lot of people were clamoring for death of ad-supported publishing on open web. Well, the future is almost here.
[0] https://talkingpointsmemo.com/edblog/theres-a-digital-media-... [1] https://www.recode.net/2017/11/29/16715350/buzzfeed-lays-off... [2] https://www.recode.net/2017/12/5/16735262/ziff-davis-mashabl...
If the market is the decider then I think the market is saying - quite loudly - the water is great to drink.
Now I feel like Methuselah, who played with dinosaurs before the Ark.
Interestingly enough, "walled gardens", like facebook, are big and important enough to bully advertisers into playing by facebook rules, accepting FB measurement standards, without calls to 3rd party vendors.
It's only open web which is polluted more and more each year.