that has a lot of traffic or is currently viral. So Youtube sends your stupid reaction video to millions more potential viewers as "impressions" vs. something you might have that was original content that youtube isn't promoting.

And 2) People click on them.

If only 3% of 1,000,000 impressions is a hit, you have 30,000 views and a tank of gas in your pocket. And it was an easy video to make, because you are ripping off another 'tubers content but by plastering your face and pausing and commenting you can claim "artistic license" with their copyrighted work.

I dislike them too, but they are successful. Most of the guys I see that are "Professional opera singer reacts to Boston's More than a Feeling" (as if they've never heard Brad Delp before in their life), are doing it mainly to get their original content promoted (and hopefully subscribers) to the Classic Rock or other vocal-enthusiast youtube audience that their original content isn't reaching. It's good advertising, despite how obnoxious they are.