Because they're both doing it for the exact same reason. Netflix doesn't want people using their service for free and neither does Youtube. Netflix didn't have ads so they cracked down on accounts. Youtube does, so they're cracking down on adblockers.

I was fine with Youtube locking their 4k+ resolutions behind premium but they're slowly tightening their hand more and more to make it 'profitable'. Hell, the queue feature is premium now. Using the app on your phone while it's 'locked' is a premium feature. Things that should be free are being stuffed into the 'premium' package but because that wasn't enough, they're trying to block adblockers. Making people pay for what they were getting for free, while it makes sense from a business perspective, never goes over well. Premium is really only worth it if you want the people you watch it get paid more, everything else can be done by third party players.

Although like Reddit, they might kill those off next.