A keeper of sheep is more somebody that gets what others produce and takes care of it. But a worker of the ground puts the shovel in the ground and tackles
the earth. He causes the earth to produce crops, something the earth would not have done without his intervention. Cain lives up to his name: a person of initiative, of intervention/action. A man. He rolls up his sleeves: we have to do more than provide care, we have to intervene. The Cains take the world forward. That is what God had commanded man to do (Genesis 1:28). There must be toil (Genesis 3:17). Cain does this, and so bread is put on the table (Genesis 3:17). Eve had seen it right. Cain lives up to what she saw in him in the cradle. Abel also puts in effort (do not underestimate that), but Cain leads the way.
2 And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a worker of the ground.