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.