Fire is the opposite of not having a work ethic. Most firees are incredibly disciplined and have a bigger work ethic than most normal people.
The result of this work ethic is to remove one’s self from the workforce. It’s a reasonable question as to why a lifetimes ambition for these hard workers is to be able to do nothing.
I think "retirement = doing nothing" is an incredibly strange assumption to make.
Most people who fire do the opposite of nothing though. For example Mr money mustache, one of the early fire bloggers, doesn't work in corporate but runs his own co working space.
I'd say most firees are risk averse entrepreneurs.