[Ajedi32]

I think this comment[1] on the linked Meta question explains it pretty well:

> To the people confused why ads need to run their own Javascript (even ones that are just static images): The short answer is that Ad Networks do not and cannot trust website operators. They need to run their own JavaScript served from their own servers in order to verify that a real user saw the ad and for how long, and they can't trust the website operator to tell them. And these pieces of JavaScript tend to be more invasive and privacy-destroying than the website's JS because they care, far more than the actual website does, that the "user" is not a bank of iphones in a sweatshop in China.

[1]: https://meta.stackoverflow.com/questions/386487/why-is-stack...