One possible way to look at it: Nobody can really change the world on a grand scale. Our limitations and non-kammic actions are dictated by the way things are. Therefore, we are more responsible for our kammic actions which represent things that are really in our hands to change.
A good kammic act will be positive for you (and possibly others) in every case, while just an altruistic, non-kammic deed can quickly be lost and at worst have contrary effects.
In philosophy, there is the idea of “heterogony”: That most changes come about by unintended consequences, and that it is not possible to intentionally change things on a grand scale, because this almost always backfires.
Maybe the Buddha thought along similar lines.