I opened youtube this morning to see a big popup:
Ad blockers are not allowed on YouTube
- It looks like you may be using an ad blocker.
- Ads allow YouTube to be used by billions worldwide.
- You can go ad-free with YouTube Premium, and creators can still get paid from your subscription
And TBH: since YouTube is not doing that, I think they might've decided the cost of that solution outweighs the projected revenue so they've chosen a cheaper solution.
In Twitch's case, more of the audience might be techy, so ad injection ended up being the more optimal option.