Luis, this is not an idle issue. This is an important metaphysical issue because rebirth, karma and nirvana are metaphysical realities and it is only logical that someone would ask Buddha what is the source of these metaphysical realities along with the existence of man and his free will to seek enlightenment in the first place. I'd like this to be addressed from informed sources if they exist.
General perception within Buddhist circles AFAIK is that those are simply not very important, to a large extent
because they are metaphysical.
You are mistaken about Karma and Rebirth, anyway. Those are not nearly as abstract issues as you seem to believe. Nor is Nibanna either, IMO.
There are many, many exceptions, many people who make those matters metaphysical anyway, because we deal with actual people who often value metaphysics and we can't leave them hanging.
You will find some approaches to the matter in Vajrayana, others in various Japanese schools.
Not all of them are very informed or advisable, IMO.