Because it ain't there.
Here: Genesis 4:
[1]
Now Adam knew Eve his wife, and she conceived and bore Cain, saying, "I have gotten a man with the help of the LORD."
[2] And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a tiller of the ground.
If to what your saying is to be true.
Then why isn't Cain in Adams genealogy as his son.
You can find Seth, but nothing about Cain, the only reason Able is not there, because it only list those who are living.
So why isn't Cain listed with Seth as Adam's son.
But nothing there
Genesis 4:1--"And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the Lord"
If you had notice, there's nothing there to indicate as Cain being Adam's son.
Only that Adam knew Eve his wife, But had you read Genesis 3:15--" And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel"
Here we find God saying to the Serpent, that there would be enmity between the woman's seed and the Serpent seed.
So who do you suppose would be the Serpent seed, Seeing Eve gave birth to Cain and Able.
1 John 3:12--"Not as Cain, who was of that wicked one, and slew his brother"
Isn't that amazing, Cain being of the wicked one, this being the Serpent.