Posted 17 February 2013 - 06:38 PM
For the same reason that when you murder someone, you are not simply allowed to do something that will bring your victim back. מעות לא יוכל לתקון וחסרון לא יוכל להמנות.
Adam HaRishon dropped an atom bomb on the DNA of the universe. He poisoned the world with the venom of the Nachash HaKadmoni. When a person does an Aveirah, it has an affect on the world, both physical and spiritual. Just as if you shoot someone doing Teshuva won't undo the damage, so too Adam could not undo the damage that he wreaked on the world with his sin.
But humanity as a whole can. We can and will rebuild the ruins of Adam's destruction iy"h, and when we do, Moshiach comes, at which point in time we take over and proceed to do what Adam failed to do.
The important rule here is, when someone does an Aveirah, the resultant damage, including the punishment and pain the sinner receives, is not an arbitrary response on the part of Hashem, the way, for example, a parent would punish their child for doing something wrong. Instead, the damage caused by an Avierah, including the punishment for the Aveirah, is a direct result of the Aveirah itself, in a deterministic cause-and-effect transaction. It's like saying that if you smash your leg with a hammer, you will feel pain. So too when you do an Aveirah, the results of the Aveirah were caused by the Averah itself, and sometimes those results are more devastating than others. And when Adam did his Aveirah, the results were devastating to the extreme.
But in any case, it is not difficult to understand that the damage caused by an Aveirha cannot be simply undone by another act, any more than physical damage caused by physical means cannot always be undone.