"On worlds where there are no regular battles they count on these to help with fireworks collections." -> Exactly !
Especially knowing in events like independence with fireworks to gather, the low attendance makes it unworth to dig : we can win as much or more while jobbing during that time, and this without having to spend HP/energy buffs, time at the hotel/barracks or stuck at the fort before the battle starts.
So even if there were players ready to dig when needed, in inactive worlds we are penalised if we dig compared to if we don't (regarding fireworks). Especially tanks. And compared to worlds with good attendance and daily battles, we can win much less fireworks in total than in this kind of worlds. Awesomia battles with extra fireworks are a good help to make it a bit easier.