Reading code is harder than writing it, and too many people don't have the patience for it.

Instead, in cases like these they'd rather invest the time re-learning the lessons of the past because they thought it should be a certain way, and if they can't instantly see a reason why it's done a certain way, then they behave (unintentionally or otherwise) as if understanding is not possible.

Especially where it's tricky it's worth including a larger pre-amble in the code itself, as well as source code control.

Often one or the other are suffering, and it's sometimes worth giving a description of how the components work together in detail, or are relied up on by more than one use case (therefore not a simple change or solution).