Limits of language, I think. Both explanations are possible, giving what the parent post said; both VAuroch and Eugine_Nier may have had experience with similar cases caused, respectively, by hindsight bias and akrasia, which makes their explanation appear obvious to them.

A lot of the time, I've noticed that "it's obvious" means "I have seen this pattern before (sometimes multiple times), and this extra element is part of the same pattern every time that I have seen it"