He made this his career. He is able to support his family and buy a house with this career. So if playing video games for 9hrs can make the same amount of money as working for 9hrs, why not?
You would be surprise how much partnered streamers make. For instance CohhCarnage (great guy and businessman) has said he has about 4500 subscribers. People pay $5.00 to subscribe to a channel and twitch gets about half. So Cohh pulls in about 11,500 a month just on subscription. This is not including tips, his website subscription "cohhilition.com", or ad revenue.
I would suggest finding a streamer you like with a decent size channel and watch it. Even if you are not into watching people play games the business side of twitch is very interesting.