There is no logic to applying an atonement before it takes place. The comparison to Yom Kippur and a sacrificial Passover lamb do not apply because their effect under the law did not apply in advance either. So if from the time of Adam people got salvation directly from God, then it can always work the same way with the eternal God. And Christian scriptures don't explain the alternative.
The problem with your thinking is that God has already given us verses saying He has done what you have said it makes no sense to do.
I will list them for your reference.
Abraham
Genesis 15:5-6
5 And He took him outside and said, "Now look toward the heavens, and count the stars, if you are able to count them." And He said to him, "So shall your descendants be." 6
Then he believed in the LORD; and He reckoned it to him as righteousness.
Romans 4:3
For what does the Scripture say? "Abraham believed God, and it was credited to him as righteousness."
Romans 4:9
Is this blessing only on the circumcised, or also on the uncircumcised? We have been saying that Abraham's faith was credited as righteousness.
Romans 4:20
Yet he did not waver through disbelief in the promise of God, but was strengthened in his faith and gave glory to God,
Romans 4:22
That is why "it was credited to him as righteousness."
Galatians 3:6
So also, "Abraham believed God, and it was credited to him as righteousness."
James 2:23
And the Scripture was fulfilled that says, "Abraham believed God, and it was credited to him as righteousness," and he was called a friend of God.
Nehemiah 9:8
"You found his heart faithful before You, And made a covenant with him To give him the land of the Canaanite, Of the Hittite and the Amorite, Of the Perizzite, the Jebusite and the Girgashite-- To give it to his descendants. And You have fulfilled Your promise, For You are righteous.
Psalm 106:31
And it was reckoned to him for righteousness, To all generations forever.
Other patriarchs were also credited with righteousness.