> why people literally do not and cannot get it right 100% of the time. Thinking so is pure hubris.

Sure we all don't get it right 100% on the first write. For this reason we test and iterate. Equally you could say that it is pure hubris that a language will enable you to make your code right 100% of the time.

You mean the vector::operator[] is an example of fast-and-potentially-UB ? I don't see any UB if the constraints are met (keeping the index within valid range).

If they are not willing in a specific case they for sure had specific reasons not to be.