This is a great question.
A scar, by definition, is not normal skin. A scar occurs when skin cells make excess collagen and other skin proteins in response to injury or trauma. Essentially, normal skin is replaced by these newly produced skin proteins, forming a visible scar.
Once the scar forms, there is really no way to remove it without causing some sort of new injury or trauma to the skin, which can then lead to another scar.
It becomes a vicious circle. The goal is to try to do the least amount of damage to the skin while treating the scar, to help diminish the appearance of the scar while not causing enough injury to make the skin react and form a new, bigger scar.
This is most evident when trying to cut out an unattractive scar. Often times, a new scar forms, which can be just as bad or worse than the original scar.
This is not to say that I advocate no therapy for scars. Quite to the contrary, I perform many procedures aimed at reduction of scar size and appearance, with many of them producing quite successful results. From lasers, to injections with steroids, to dermabrasion, and even surgical scar revision, there are many things that can be done to improve the cosmetic appearance of a scar.
But making it vanish forever - not possible (yet).