> why is my browser not similarly circumventing this protection?
Because your browser also runs the javascript that displays the ads (unless you've also blocked them, which I think could also be considered a "circumvention measure", IANAL).
Because your browser also runs the javascript that displays the ads (unless you've also blocked them, which I think could also be considered a "circumvention measure", IANAL).
That would work in a livestream very well, but not in a video. Imagine everyone getting adverts in the voice of live streamer in the different times of the livestream, and live chat getting time-dilated to compensate, without the streamer itself noticing.