Since rationalizations are usually employed to repair cognitive dissonance, and cognitive dissonance is strongest when image preservation is necessary, one hypothesis would simply be that these people have self-images that don't need much preserving.

Possible test: do these people have an unusually high tolerance for situations and/or self-disclosures that most people would find shameful, humiliating or embarrassing? This might explain a lack of need to rationalize, regardless of the reason for the high tolerance. (For example, we should expect sociopaths to not see any need to rationalize their actions to themselves.]

[Edit to add: not to imply that any of the people in your experiences are sociopaths; just noting that it's another situation where somebody would have a low need for self-image preserving rationalizations.)