In recent years, watching others play video games evolved from a niche activity into the global phenomenon known as esports. It now boasts professional leagues, tournaments, and major sponsors. Sixty-three million Americans watched esports league games in 2018, according to a recent survey by Activate, matching the viewership of NBA games.

Chart showing 2018 U.S. sports viewers by league

Data source: Activate. Chart by author.

Furthermore, Activate believes that esports could hit 84 million viewers by 2021, putting it in second place after the NFL's 141 million viewers. That's why Disney's ESPN now covers esports alongside football, baseball, basketball, and other "traditional" sports.

That projected growth also explains Amazon's (AMZN -0.18%) acquisition of Twitch, the top game-streaming platform, for $1.1 billion in 2014. Twitch now hosts over 15 million daily unique visitors who watch games for an average of 95 minutes per day. Over 2.2 million gamers stream their content on Twitch monthly, with the top 5% considered major influencers.

Check out the latest Amazon earnings call transcript.

Three games consistently dominate esports viewership, according to Newzoo: Valve's Counter-Strike: Global Offensive and DOTA 2, and Tencent's League of Legends. All three games are at least five years old, but they attract casual viewers with easy-to-follow rules and combat -- strengths that newer games often lack.