1. They can't afford the Cobra (insurance) plan.
2. Identity. Most people define themselves by what they do.
3. Power. Seniority. Control. You give that stuff up when you retire.
4. Social interaction. At work, you see and interact with other people every day.
5. No desire to do other things. It's easier to explore other options when you're in your 20's & 30's. It's harder to do those wild things when you're older.
That said, some people need to retire when they're 25, and they don't. So, what're ya gonna do?
Add "The like what they do" to your list and I think you nailed it for most people.
I'd say #4 will likely be my reason if I work past retirement age. My parents retired nearly 20 years ago and still do a remarkable job travelling and interacting with their friends. My dad keeps saying that the minute he sees his friends "slow down", they die. Kinda like their reason to live just leaves them.
It's easy to say you'll have hobbies to keep you going, but if you're in your 30's or 40's now, you have to realize that you likely haven't lived half of your life yet.
Life isn't short. It's the longest thing you do.