Number of reasons:

1 - Move to multi-player. I have zero interest in spending my free time dealing with humanity at large. It's not so much that I can no longer compete with twitchy teens who do nothing else (although, obvs, that's true), it's the cheating and the abuse and and and...it's just not fun.

2 - Packing games with "hours" of "content". Oh yay yet another fetch quest. Awesome I have to spend hours collecting herbs! Yay I should replay this level nine times to get stronger! If there were more games like Portal (a tight/short experience) I'd play more but grinding is boring and I don't play to be bored.

3 - Micro-transactions/pay to win. All the nope. Not doing it.

4 - Seen it all before. One of the realities of growing old and, frankly, why "we" end up cranky is that very often if you have more than six brain cells you can see every twist, story beat, game mechanic etc.. coming from a mile away. What you find fun and innovative at 16 is often really boring and predictable when you hit 40.

5 - Less patience for things that don't line up perfectly. Like when I read that BOTW had weapons that explode every eight seconds I was out. In 1985 I'd have played it regardless. My ability to deal with game mechanics I don't enjoy is next to nil at this point.

6 - Tired of getting screwed. The Borderlands 3 thing is a classic example. I am not downloading another launcher so I just won't play.

7 - All the classics others have mentioned (mainly time).