That this is generally a problem, and was established as such before software development existed (the big thing people usually point to is a RAND corp from the 1940s) and is the whole motivation for Wideband Delphi estimation methods invented shortly afterwards (of which agile "planning poker" is simply a particular more recent realization) for forward estimation, and why lean methods center on using a plan-do-check-act cycle for process improvements rather than seat of the pants and subjective feel.

But despite the popularity of some of this (planning poker, particularly; PDCA for process improvements is sadly less popular) as ritual, those elements have become part of a cargo cult where almost no one remembers why we do it.

That doesn't surprise me at all. Isn't software engineering in essence about being constantly confronted with new problems to solve and having to come up with a sufficient one on the fly? It seems very hard to estimate this, even if you know yourself well.