This is mostly a matter of multitasking (which I know HN is fairly divded on).
I play games when I watch TV, and I also use games to socialize with my friends.
If an average person added up ~80% of their socialization time and all of their TV/movie/audiobook/podcast time, I suspect that it'd look pretty close to the amount of gaming I do when I have a full time job (probably fluctuates between 10-40 hours per week).
I still enjoy games, but with adult life I'm playing either after diner (if I'm not too tired) or on the weekends, and gaming competes with other entertainment (series, movies, going out with friends/dates with SO, etc). So as I've grown it takes less time overall.
I am, however, making a schedule for Homeworld 3 and KSP2 when they launch (: