I think you have a set of older folks out there who have seen their peers retire themselves into irrelevance. Although there is a decent percentage of people on this forum who are FIRE, most of us are in various stages of the FIRE rat-race (yes, it's still a rat-race, no matter how we slice it), so it's hard to imagine what happens once we're there. I think once the novelty of being FIRE wears off, there's this "ok, now what?" moment. Many reach FIRE when they still have kids in the house, so that's a wonderful and rewarding avenue for our time. Home renovation, travel, etc, etc. But, what happens when the kids are in college and you've checked all the major boxes on your life TO DO list? My guess is you try to find societal relevance. Either with a job, volunteer work, or some kind of "career" that defines you and establishes a legacy in society.
Just some random morning ramblings!