The card text reads: "Reroll all dice"

Why are dice placed on character cards not considered "all"?

Because they haven't been rolled, so they can't be rerolled.

More specifically looking at the rules for discarding a card and rerolling dice it specifically says the dice need to be in your dice pool. Your dice pool is comprised of all the dice from cards you've activated (rolled). If they aren't in your dice pool, they can't be rerolled.

Unless otherwise noted on a card, I think it's safe to infer that all references to rerolling apply only to the dice your pool, in this case the "all" would include your opponent's dice pool as well as yours.

Dice are essentially considered "out of play" or "removed" if they're not in a dice pool (yours or your opponent's).

Disturbance in the Force rerolls ALL dice, meaning dice in both your and your opponent's dice pools.