Why is it not enough for Jesus to be human to be the perfect sacrifice, why must he be God? Regardless of whether Jesus is God or not, why do some stridently hold to, 'Jesus can only cover sin by His own death as God'?
Until Jesus' perfect sacrifice/offering, the law required an animal to remedy the debt of sin each person/Israelite had before God. An animal was killed, blood was spilt to cover the sinner's guilt so they be 'cleansed' and made right with God again.

Your lamb must be an unblemished year-old male, and you may take it from the sheep or the goats. Ex 12:5

Paul explains a lot about Christ's sacrifice being sufficient to cover all sin - by him faithful believers are made free from the debt of sin - past present and future. Naturally, all mankind is covered by this sacrifice except the few that may fall outside for some reason or their own choosing.

For by one offering He has perfected for all time those who are sanctified Heb 10:14


But now He has appeared once for all at the end of the ages to do away with sin by the sacrifice of himself. Heb 9:26


we have confidence to enter the holy place by the blood of Jesus, 20 by a new and living way which He inaugurated for us through the veil, that is, through His flesh 10:19-20


He learned obedience from the things which He suffered. 9 And having been perfected, He became the source of eternal salvation for all those who obey Him Heb 5:8

Jesus is holy and without sin why would this human only sacrifice not be enough?
EDIT:
I thought it would be assumed that Jesus is holy and sinless because his Father is God (as scripture provides). Thus he has no inherited sin, and is not a just a 'good' man. Clearly any man apart from Jesus would not suffice.
God gave detailed instructions on how and when animal sacrifices were to be offered to cover sin.  The underlying principle is that blood has to be shed for the forgiveness of sins:

Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sins (Hebrews 9:22).

This was to continue until Christ came to offer the ultimate, perfect sacrifice, which made animal sacrifice no longer necessary.

But those sacrifices are an annual reminder of sins, because it is impossible for the blood of bulls and goats to take away sins (Hebrews 10:3–4).

Yes, Jesus is holy and without sin, so why would a perfect human sacrifice be insufficient if he was only a man?
One human life, even the life of a perfect man, couldn’t possibly cover the sins of the multitudes who ever existed.  Neither could one finite human life atone for sin against an infinite God.  The only viable sacrifice must be an infinite one, which means only God, manifest in flesh, could atone for the sins of the elect.  Only God, an infinite Being, could pay the penalty owed to Himself.  This is why God became manifest in flesh to dwell among men (John 1:14).  No other sacrifice would suffice.
God the Son sacrificed Himself to God the Father and thereby fulfilled all the requirements of the Law.  Unlike the temporary sacrifices, Jesus’ once-for-all-time sacrifice was followed by His resurrection.  He laid down His life and took it up again, thereby providing eternal life for all who would ever believe in Him and accept His sacrifice for their sins.  He did this out of love for the Father and for all those the Father has given Him (John 6:37–40).
Sin entered the world through Adam, and Adam’s sinful nature has been transferred to every baby born into the world (Romans 5:12)—except for Jesus. Because Jesus did not have a human father, He did not inherit a sin nature. He possessed the divine nature from His Heavenly Father.
Jesus, the Word, became flesh in order to dwell among us (John 1:14). Jesus came (from heaven) to give his life as a ransom for many (Matthew 20:28; Mark 10:45).
Jesus was the final, perfect sacrifice that satisfied forever God’s wrath against sin (Hebrews 10:14). His divine nature made Him fit for the work of Redeemer; His human body allowed Him to shed the blood necessary to redeem.  No human being with a sin nature could pay such a debt.  No one else could meet the requirements to become the sacrifice for the sins of those whom God had chosen, the elect  (Matthew 26:28; 1 John 2:2).  If Jesus were merely a good man as some claim, then He had a sin nature and was not perfect.  In that case, His death and resurrection would have no power to save anyone.
Because Jesus was God manifest in flesh, He could pay the debt we owed to God.