Dota 2 - International 5 tournament prize pool reached more than 18 million dollars

People also bet skins on live matches - CSGO Lounge - Marketplace, Trades, Bets

It does boggle my mind when I see 12,000 people watching a Twitch stream and all the guy is doing is opening cases (for skins) for 2 hours.

That's why all the big game studios have psychologists on staff integrated into the entire dev process.

Valve
Psychologist job opening at valve:
In order to create exceptional products that people will use and appreciate, we need to know about human behavior and about the motivations and influences underlying how and why people do what they do.

We want to exploit your experience with experimental design, research methods, statistics, and human behavior to help craft even more compelling gameplay experiences for future Valve titles.

Duties:
-Provide relevant insight into human behavior in order to shape gameplay and customer experience.

-Design experiments to evaluate various gameplay hypotheses and design choices.