No brainer for the majority really. As you grow older, you lose not only the free time you have to play (due to work or/and family) but as you grow past 30, you also start to lose that "competitive fire" in you to play a game just to get good at it against other people. Your focus simply isn't the same.
Oh, and one more thing that should be noted and quite strangely I haven't found anyone mentioning here... Money. Maybe back in the teen/kid days you had your relatives buy you the games, but as an adult, it comes out of your pocket, so you really have to choose wisely which games you are going to dedicate your free time to.
But I personally (I'm 28) never liked online in any game ever. The only exception to this was Counter-Strike (from 1.6 to GO), which I've played between 2002 and 2020, and mostly because I've made a lot of good friendships from it, and the game was really addictive for some reason. Dropped it already because of three reasons, time constrains, the game no longer being fun and the people I hanged out with leaving.
Sports games? Those are done for me, since a long time ago. Same goes for yearly shooters like COD, Battlefield and arcade car games like Need for Speed. Unless it's a remake of Need For Speed: Most Wanted 2005. I'd play that **** again for sure.
Since about 2016 or so, my main focus has been single player games, and mostly games with an actual narrative to them. movie-esque games, like God of War, Horizon, The Witcher, Last of Us, etc.
Gran Turismo has been the only exception so far because I am a guy who loves car culture and I don't think any other game displays it better than GT. Not to mention the Campaign, which is the closest thing you can get to a story-telling car game while having actual decent physics and not the arcadey kid physics.
Only times I play online is to get achievements (trophies) and nothing else. I'm a bit of a completionist.