The probability distribution function of average coder understandable-documentation decays exponentially with subject matter expertise, in most cases. Furthermore, it is often semi-rational, usually unconscious non-/for-profit "job-securitization" to lay landmines and obscure functionality to reinforce in-group, elite, "arcane knowledge" hoarding and seem "expert."

A startup founder often wants to work themselves out of a job to do more useful things, most will do all they can to insulate themselves into indispensability (a lot of technical people are super insecure, I'm proly one of them, and a lot of corporate gigs are cut-throat fiefdoms).