If Guatama Buddha did not concern himself w/afterlife, then why is the idea of karma so central to Buddhism?

Karma, a central tenet of Buddhism, entails the notion of rebirth. This is confusing to me because Buddhism as I understand it, is less concerned about explaining the "big questions" like where we came from and where we go when our bodies die are more interested in practical ways in which we can enrich,... show more Karma, a central tenet of Buddhism, entails the notion of rebirth. This is confusing to me because Buddhism as I understand it, is less concerned about explaining the "big questions" like where we came from and where we go when our bodies die are more interested in practical ways in which we can enrich, enlighten ourselves and make sense of THIS life. If the religion spends so little of its effort attempting to explain or predict an afterlife, why (for lack of a better way to put this) are Buddhists so sure that they will be reincartnated? Forgive me if I sound ignorant.. I'm learning.
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