They can drop. Rolling a GC is essentially trading the charm for another randomly modded charm dropped by the same monster (or boss). Since it is more frequent for people to get a good charm by taking a GC dropped by Baal and rerolling it (thus forcing Baal to 'drop' a few hundred different grand charms for them, until they get something they want to keep), it is more noticeable to see/hear of people getting good charms via rolling as opposed to random drops off Baal.
Think about it - I kill you and you drop a charm for me. The chances of it being a 40+life skiller are very small, but If I take that charm and simulate you dropping a few hundred grand charms for me, my chances of that charm being something good are, statistically, improved.