No doubt, Cain wondered the same thing and when it was time to bring sacrifices to God, he brought a bunch of crops and produce, whereas his brother, Abel, sacrificed his finest livestock (Genesis 4:2-7). God accepted Abel's sacrifice, but not Cain's. Cain couldn't understand why his sacrifice was found lackluster and unsatisfactory.
So what did Cain do?
Why, he not only drew his brother's blood, he killed him outright (Genesis 4:8). Tell me, if Abel would have offered Cain a bunch of crops and produce to NOT kill him, do you think it would have changed Cain's mind? No, it wouldn't have. So, if a sinful Cain who was unjustly angry would have refused an offering of crops and produce, why should a righteous God have been moved by HIS offering of crops and produce for his sins?
Do you see how Cain made himself look foolish here? Can't you picture him standing there trying to tell God that blood sacrifices were barbaric while he has his brother's blood all over his own hands? That's exactly how erroneous WE look when we try to pose a morality test to a righteous and all-knowing God. If you walked into your house to find some stranger molesting your children, what would you do to that guy? You'd beat him into a pile of bloody hamburger, and you'd be justified. If that's what WE would do, what do you think a righteous and all-knowing God that tolerated no sin AT ALL would do?
THAT, folks, is why Jesus died a bloody death to atone for our sins. We are all guilty of deadly sin whether we admit that in this life time or not, and without the blood of Jesus speaking for us, we're toast.
And almost all things are by the law purged with blood; and without shedding of blood is no remission.
For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul.
How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him;