Why do people want to retire at 45-50 when they're probably still going to be very capable?

Why do people want to retire at 45-50 when they're probably still going to be very capable?<br><br>&#x200B;<br><br>I don't get it. I'm 17 and currently studying. I know at 45 I would probably be much more capable than I am now. Why would you want to retire when you're at your peak?