For certain, part of it is that adult life gives me far less time to enjoy "dedicated" gaming. I have plenty of time for mobile games on my commute (I walk), but I no longer have dozens of hours a week that I can dedicate to CS or Eve Online.
But part of it is that the Steam store is anxiety-inducingly overwhelming. It feels like there's more than a lifetimes worth of content in there, and I have absolutely no idea how to find the good stuff. Something about Steam's design appears to provoke this response, because I do not have similar issues with the similarly large Apple store. I think a redesign might be in order.
And the recommendation algorithm just keeps sending me the same games that don't look interesting to me.