To build a folly is essentially to do something a second time, something at an inopportune moment. That something is always the memory of something forgotten, about which we can paradoxically say "There it is again."
Follies were misunderstood, purposeless constructions. They were often only small, extravagant gestures in a garden, easily whisking off the imagination to distant lands, a sort of time capsule built to awaken the memory and induce surprise in passers-by. They marked locations, organized secondary paths in a park, or simply predicted the arrival of better times—a demarcation, a sacred spot, a mysterious trail, a hill whose tragic rocky nature begged for a tower, a party, or the arrival of summer.
“It is demonstrably true that things cannot be other than as they are. For, everything having been made for a purpose, everything is necessarily for the best purpose.” — Professor Pangloss
Sometimes there’s a Heuristic That Almost Always Works, like “this technology won’t change everything” or “there won’t be a hurricane tomorrow”.
And sometimes the rare exceptions are so important to spot that we charge experts with the task. But the heuristics are so hard to beat that the experts themselves might be tempted to secretly rely on them, while publicly pretending to use more subtle forms of expertise.
…Maybe this is because the experts are stupid and lazy. Or maybe it’s social pressure: failure because you didn’t follow a well-known heuristic that even a rock can get right is more humiliating than failure because you didn’t predict a subtle phenomenon that nobody else predicted either. Or maybe it’s because false positives are more common (albeit less important) than false negatives, and so over any “reasonable” timescale the people who never give false positives look more accurate and get selected for.