Twitch is live streaming which is what probably makes it a challenge to block ads, and the main draw of twitch is watching live content. I’d imagine it’s easier to view content that isn’t live without ads, and people do repost clips after it’s aired where people haven’t encountered ads in contrast to live viewers.

Then look at television piracy where live viewing will have ads, but pirated content is uploaded with it stripped away. Blocking ads will be something YouTube will have to keep fighting endlessly.