Check this out.
https://zakird.com/papers/tangled_web.pdf
A typical web page today has numerous external dependencies (both objects and executable scripts), often for the purposes of displaying advertising. They reported the LA Times home page pulls 540 resources from nearly 270 different IPs, spanning 8 different countries.
As a publisher, when you include some simple "ad code" in your site you never really know what's going to end up loading or executing...