Because he is also flesh
Genesis 6:3
Then the Lord said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.”
Why would God not strive with man forever because he is flesh? He was flesh the whole time. That doesn't make sense at face value.
The phrase "because he also is flesh" is actually only two words in the original Hebrew text.
- shaw-gag' (Strongs 7683)
- to go astray, err, commit sin or error
- baw-sawr' (Strongs 1320)
- flesh
That first word, meaning to go astray, is only used in 5 verses. After reading these verses it becomes apparent that this word is used to describe unintentional sin.
Genesis 6:3
Then the Lord said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.”
Leviticus 5:18
He is then to bring to the priest a ram without defect from the flock, according to your valuation, for a guilt offering. So the priest shall make atonement for him concerning his error in which he sinned unintentionally and did not know it, and it will be forgiven him.
Numbers 15:28
The priest shall make atonement before the Lord for the person who goes astray when he sins unintentionally, making atonement for him that he may be forgiven.
Job 12:16
With Him are strength and sound wisdom, The misled and the misleader belong to Him.
Psalm 119:67
Before I was afflicted I went astray, But now I keep Your word.
It so happens that the same word in its female form (sheg-aw-gaw', Strongs 7684) shows up in the Bible 20 times. The Strongs definition of the female version of this word is "sin, sin of error or inadvertence", making this view of the word even clearer. This version of the word is translated as "unintentional" in English 14 out of the 20 times it is used and some of those times it is actually used to modify the more general word for sin (Strongs 2398).
So the phrase in Genesis 6:3 is communicating an unintentional sin involving the flesh, particularly implying having been mislead. It's as if God is saying "They don't know what they've gotten themselves into, but I need to stop this. Nevertheless, I will give them 120 years to turn around before I destroy everything."
This brings to mind the first book of Enoch, which portrays the majority of the blame being justly placed on the shoulders of the Watchers. It describes the Watchers as giving humanity forbidden knowledge and teaching them evil ways. Yet just as Adam was still at fault for eating what Eve wanted to share with him, humanity still bore part of the blame and they too would perish in the flood soon to come.
I am not suggesting that the translation "because he also is flesh" is incorrect. I am suggesting that the weight and nuance of that phrase isn't always obvious, and digging into the original Hebrew language helps us reconstruct that deeper meaning. This word is generally used to convey a sense of misleading. This misleading could be the words and actions of the Watchers and their offspring the Nephilim. Either way, this verse conveys God's justice once again being balanced with mercy and an understanding heart towards His creation.