"For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul?" Leviticus 17:11
Because the penalty for sin is death. The sinner deserved the death penalty. Therefore, when the blood sacrifice was made, it was an offering of life itself in substitution for the life of the sinner who deserved to die.
Of course, human sacrifices were prohibited, because another human would also have blood which was contaminated by his own sin. Therefore, God ordained animal sacrifices. Animals are not made in the "image of God," cannot reason about right and wrong, and therefore cannot sin.
This clean blood of a clean animal was, however, could serve only as a "temporary covering" - it could not actual atone for (remove) the sin. So, the animal sacrifice had to be repeated over and over again. This was God's method throughout the Old Testament.
In order to permanently solve the sin problem, only the blood of the sinless Son of God (God in the flesh) could remove sin, and that is known as atonement. "Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us" (Hebrews 9:12). "For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not die, but have eternal life." John 3:16.
There is no death for the believer. He is resurrected, just as Jesus Christ was ... into eternal life. Only God could come up with a plan to solve the sin problem, and only He could carry it out. And He has freely offered us payment for our sin - and eternal life.
All scientists understand that the blood is the life. William Harvey, in the 17th Century, discovered that biological life really is in the blood, which provides nourishment to all parts of the body and carries away its waste. The Bible is the final authority on all issues of science and will always be proven true. Only the Creator of the cosmos could have all the answers.
If you have questions about the Bible, ask the Lord to reveal the answers to you, and study His Word. He will reveal it to you.