Because it’s impossible to block adblocking. The server can’t know whether the client plays the video. The best they could do is have you wait the ad-time even if the ad is blocked but that would just mess with their analytics - they want to be sure the ad is being watched.
The only reason adblock blocking works for smaller websites is because adblockers need to catch up with each implementation. People will easily catch up with Youtube as there are thousands of people working on Youtube programming.
The clever check is impossible. The server can’t know the client is showing the video even when playing DRM content. I could literally mute the sound and put a black box over the ad until it’s over. The problem is as old as the internet itself.
That’s why Apple and Meta pushing eye tracking so hard. It’s the only solution to the ad blocking problem.
I think YT is doing well with YT premium and that’s the way to go for them. It’s one of my favorite subscriptions and I’d probably stick around even if they raise the price unless they hurt the creators so badly they bounce and I’d bounce with them.