You're already trusting an entire browser, with all its transitive dependencies, a large portion of your OS, and probably also every single program you run with the same uid as your browser.
But an extension requiring explicit permission to observe and modify all of the content your browser ever renders is unacceptable.
It's sort of a separation of concerns. There's no fine-grained way to say "intercept this list of URLs and deny some of them" without allowing it to also view and modify the entire content of every web page.
But an extension requiring explicit permission to observe and modify all of the content your browser ever renders is unacceptable.