In that case, why didn't he just reseal the bore? Or even better yet, come down and prevent it from happening? If you read the EOTW prologue, Ishamael tells Lews Therin that the battle between good and evil has always been decided by humans. Ishamael is the Anti-dragon, if you will. Why does the dark lord want the Dragon to convert? Because it is the ultimate victory. Not only will It be released into the world, It has corrupted the champion of the Creator.
Ishmael also stated that in ages past the Dragon has fought on the side of the dark lord. Although, can we really rely on info from one known as the betrayer of hope. But, the way Herid Fel explained it to Rand that might actually be posible. Herid told Rand that the Bore exists outside the Wheel and therefore will always exist once the Wheel spins around to that point again. If you believe Ishmael, once upon a time the Dragon fought for the dark lord and yet the world still exists and once again the Dragon Reborn is back to put things right. So, maybe it's kind of like parrellel dimensions. After each decision a new dimension is spawned ever increasing the number of dimensions to infinity. So why would the creator force this battle to continue endlessly through countless ages and dimensions you ask? Maybe that is the point of existence, for good to struggle against evil for all eternity and never really having a winner in said struggle? Or is it the fact that GOOD continues, at all, to struggle against evil that makes the Creator the victor? Ah, who know's what motivation any deity has for doing anything....It is fun to ponder though.