And if a Leaguer is handling a disaster they still are required to deal with it. Like do really think it would be in character for Aquaman to be saving a bunch of people from tsunamis to suddenly drop everything and "welp, see yah, I am going to fight the big thing in sky have fun drowning". No, he would stay and continue to help save people. That's basically (in terms of plot mechanics) what every Leaguer not on screen was likely doing, something that concerned a life or death situation that they could not be torn away from. Its not like a firefighter immediately abandons putting out a small house fire just because they have been told there is bigger house fire elsewhere.