And why should they? I'm no fan of trust fund babies who coast by on life, but really, why does anyone have to justify their lives by doing something to improve the world? What are you and I accomplishing right now?
Maybe it's just the severe burnout talking, but I for one would love to just stop everything and exist for a while with no tangible goals or requirements of me. Besides that, what good is a person who did the leanFIRE thing really supposed to do? The whole idea is to live cheaply, they don't have a lot of money to invest in making the world better. They have all the time in the world, sure, but if you're putting 40+ hours into "making the world better" (Whatever that means to you) then are you really retired? Or are you just doing the same thing as everyone else but living a much more meager life for the sake of it.