why? because it's more complicated than that.
sin by Hebrew definition refers to a moral debt, and debts need to be paid sooner or later. since everyone is accountable for their own sins, everyone has to pay. the wages of sin is death, so you had to die for your own sins. animal blood was a temporary payment for sins as you were going to do it again, basically guaranteed.
why does blood pay for sins? I don't know. it's refered to as the "lifeblood" in Hebrew. Jesus, having God's own blood, being His Son, had the blood that could potentially pay for all the sins of every human being who accepted Him as Lord and Savior. since blood was only the payment through sacrifice, accepting Christ is like giving that sacrifice.
Jesus' human body was just that: a human body. Jesus on the inside, His soul, was God, but His body was still mortal. He became man; He lived to die.
again, human. I'm pretty sure losing 2/3 of your blood, being forced to manually lift your own cross-beam and then being sent up on a cross with your lungs collapsing, punctured side, flogged back and impaled skull would kill anybody.
the God part of Him cannot die. the human part, just a tool, was dead. that's all your body really is; a tool for your soul to live on earth.
no pain? He said Himself that as a man He felt rejected by God (the other 2/3), He was spit at, He was tortured, flogged, mocked, had thorns driven into His head bruising the bone, nails in His hands, bleeding to death, He was rejected by men especially those of His own religion, had a splintered cross, His friends deserted Him, He could see His mother crying, lungs failing, people dividing up His clothes....
like I said, He had a human body. if I slapped you, wouldn't you feel it?
human. you're not grasping the concept very well, are you? Allah is more limited than the Trinity. does that mean Allah is imperfect?