In mirroring WIP's reply to your post, Jesus was, is, the only perfect Lamb, the holy sacrifice, that would be worthy enough for God. No other alternative would do.
Imagine if you owed somebody 100 Trillion dollars and the only way to repay that debt would be for you to pay them back the exact amount, down to a penny. Jesus is like that 100 Trillion dollars you, me, and everyone else in the world cannot ever repay out of our own pockets.
You could ask, why didn't God just forgive everyone by snapping His fingers? He certainly could have done so if that is what God wanted to do, but the fact that He didn't do it that way is quite revealing of His character! Our God is not the God of confusion but of peace (1 Corinthians 14:33), and I think it is safe to say that God is the God of order and orderliness. If you look at the Bible closely, you'll notice that God is meticulous every single time. As THE Author of the greatest narrative, God infused His creativity into the art of storytelling, and decided to write the most amazing epic tale by sending the prophets first and then later, Jesus, His one and only Son, to save the world (John 3:16).
His ways and methods are often mysterious and we may not fully understand them until we are in Heaven, but I believe that by sending Jesus into this world, God purported to reconcile sinners to Himself in the greatest fashion possible.