One of the design goals in RQG has been to incentivize passage of campaign time. An adventure a season (or so), means that the gamemaster can let the setting develop, and have the players respond to those developments.
So for example, if our campaign starts in Darkness Season 1625, we have about five adventures before the Lunar Army invades again and the throne of Sartar is once again vacant. Our players have likely built up enough wealth and contacts to get caught up in the fears that vacancy creates.
We get another five adventures or so, and then Argrath lights the Flame in Boldhome. We get lots of other people doing things, and once again the players get plenty of opportunities to get engaged in that. We get another ten adventures or so, and we might find ourselves on the battlefield where the Red Emperor and Jar-eel the Razoress are on the other side.
By then we are twenty adventures in. There’s a good chance are party includes Rune Lords and Rune Priests, and we’ve gotten ourselves into the Hero Wars. The setting has changed and we are long past raiding cattle or delving the ruins of the Big Rubble, but the setting has changed with us.