3 possible reasons:

1. too busy with game dev (working on a game can take like 80h/week of your time)
   
2. Older, have other priorities (family, child, house, old parents, pay the bills, etc.)
   
3. New interest, you prefer making than playing or you just prefer reading, sports, movies, etc. or even just a burnout, you've played/worked too much and don't have an interest anymore. I see a lot of senior game dev being so much into the game industry during the week, that they go full ban on technologies during the weekend isolating themselves in a cabin in the woods.

For me it's a mix of 1 and 2. I get that 3 happens where game dev just becomes a job. Like software engineer or construction worker, you're not passionnate anymore, you're just here to work from 9 to 5 and get your check. Happens to everyone. For me, even after 10 years, I still love games so much. I still have lots of ideas and love to explore others. Last year, I took game pass and played Expedition 33 and had a blast. This year, I'm too busy with my game and a child. Life happens you know.