Well popular argument from Christians would be, God has rules, and He prefer to keep it. And this preference is very important otherwise would be non Godly.
Same with the question why did God let Eve took the fruit, there's something in freedom that makes it void if God were to limit everything we do by His power. He decided that the thing that we call free will works in this world.
Therefore in order to keep God's laws, some procedure must be taken in terms of saving humanity. And apparently that procedure was to come into the world as a human.
If God just choose to make an instant change and make it so that humans didn't committed the sin, then would it be the same human as before? No it would be something else. And what happened to the previous humans, just vanished? God could, but God wouldn't because He love his works.