Grokking that 'why' can take non-trivial mental effort by a non-author, even when well coded/documented. Worse, if the code is needlessly complex, or trying to be smart or over-engineered, any amount of commenting wont help. The non-author (maintainer) of the code is now burdened. And if (so commonly happens) - they dismiss the original code as 'non-performant' or 'not a best practice' or something else.. we know how that plays out.