Ads used to be run at the streamers' discretion, and they were beaten by adblock. Now adblock doesn't work on Twitch, because they did the smart thing and embedded them into the stream. Also, a few years back, even though streamers have an incentive to run ads, because they benefit from it too, Twitch implemented mandatory thresholds for number of ads that need to be run or else you lose access to some tier of monetization, so most streamers leave it on auto pilot now. It means that whenever the same stream is running on YouTube, I'm watching on YouTube so I don't miss anything.