More on animal sacrifices (Hebrews 10:1-18)

The first part of Hebrews 10 continues the theme of animal sacrifices.  A few statements stand out to me:

God did want or desire animal sacrifices.  (verse 8, citing Psalm 40:6-8).  Likewise, it was "impossible" for the blood of animals to remove sin.  (verse 4).

This stands out to me because it suggests that God knew the Old Covenant would be defective all along.  Despite that, God used it.

Why would God do this?  I don't know.

Here is one idea I will throw out now, is that actually honors the traditions at the time of the Mosaic Law.  Abraham and the other patriarchs performed animal sacrifices, which God honored.  How did Abraham know to do this when the Mosaic Law's prescriptions came down afterwards?

I don't know.  Perhaps it stems from Cain and Able, who gave offerings to God.  However, in Genesis 4, I cannot find a directive from God about sacrifices.  It seems that Cain and Able voluntarily decided to do it.

I am not sure where the idea of animal sacrifices first arose.  It seems to have been a common ritual at the time, which God honored.

Another thought on the impossibility statement in the text.  If people could not afford animals, they could offer the grain offerings.  Obviously, grain does not have blood.

Looking at all this collectively, the Old Covenant sacrifices were never perfect.  But yet, God had a relationship with the people.  What does that say about God?  To me, it shows God's mercy.