It is not their fault that they are so shortsighted really. They are still young so they haven't learned to have a perspective that considers far-reaching consequences. Even now, it is still difficult for them to fully understand the ramifications of the war on them.
Fortunately, there are some things that are very easy to consider and understand right in front of them. Those things are short-term benefits but it is enough for them to question whether they would be involved in this war or not.
Someone asked in agitation, "Are we just going to watch as an army of millions just passes us by without doing anything?"
The Celestial of Light and Justice had expected that the other Supreme Beasts wouldn't help the shadow of Despair because of how he treated them earlier. The Celestial was right but he didn't consider that some of them were tempted to intervene. He didn't consider that they might intervene to help themselves.