Because it's usually mandatory if it's a big company, and people doing a job they care about don't usually want to stop.
No it isn't, and people who do a job they care about know when they're not able to do it as well as they could when they were younger.

I don't think you're old enough to really understand this. very, very few people can do a job forever ho matter how much they love it. There are lots of really important jobs that anyone would want but still wouldn't do forever unless they felt obligated to.

I've been in game dev and I want to do it, but will I still want to do it when I'm 80? No, I'll probably want to enjoy those golden years, so to speak.