Maybe other peoples developed differently, simply because they left - and were exposed to different challenges, as it were. Or maybe those that left self selected, based on a gene mutation prior to leaving.
Who knows.
Maybe they left because they got tired of having to take care of and feed those that didn’t plan ahead.
Those that consumed their food all at once and only wanted to feed themselves on a day to day basis, and if they couldn’t feed themselves on a day to day basis they simply took food from others that did plan ahead and didn’t consume their food all at once.
Some of the tribes in Africa are still that way. Hunt and gather enough to feed themselves for a day.