I suspect it to be far less rare than you think. Literally every human at some point in their life asks "What is the purpose of my existence?". They are then faced with some variation on "to be or not to be." It is not the exclusive domain of the idle and the educated, they're just the ones who write about it. It's a universal condition, and effects those without leisure equally. If that weren't true religion would not have been the driving force behind much of civilizations rise. Even the overworked and overwrought look for answers. Perhaps ESPECIALLY the overworked and overwrought. So maybe they did not sit about in cafe's discussing it, they mined coal while thinking it quietly to themselves or shouted it in crowded bars after work when they'd had a bit too much to drink.

Hell, I was a teenager working at Taco Bell when I first read Camus' "The Myth of Sisyphus". Well before that though many a bean burrito were handed out the window while I considered the absurdity of life. Perhaps my thinking wasn't as nuanced as it would later come to be, but the concepts occurred to me quite naturally.