It's more like this, at the point the quests which require flying become available, you have had all the possibilities to get the flying. All the quests that require flightng are in the "second wave" of quests of a zone, when you have, if you'd followed the questline (main and side) gone through the zone already, done all required quests and had reasonable chance to pick up all he currents.

Those who stumble to the fly requiring quests before they have flying tend to be those who cherry pick what quests they do, and ignore getting the currents when it's easiest, that is, just picking them up along the way when doing the first wave of the quests of the zone.