I think the long-standing MOBAs such as League and Dota have a unique advantage in this area.

Many people grow frustrated with playing the game themselves, with most games being ruined by uncooperative crybabies or just straight trolls. Losing at 10 minutes but being held hostage in the game regardless is incredibly frustrating.

You don't have these issues while watching someone else play, or watching a tournament. So its become quite common to continue watching these games even if you barely ever play them yourself.