eSports betting is one of the emerging alternatives aside from online casinos. I think what you refer as gaming here is eSports as video gamers can compete via online format. And this industry is already multi-million so I believe there are a lot of gamblers who will explore this option.
Not everybody is a fan of online gaming, I have tried some of my co-workers and friends to bet or try to play online game such as Dota2, but they just don't want to switch their gambling experience. They all loved betting with sports. Though, I agree that the eSports industry is generating millions of prize pool and income, thus attracting big time investors and gamblers to engage in eSports. The same reason why there are several TV channels available for eSports already.
There's just people who doesn't like online games.