You're aware that not all the Volm would have left Volmworld to chase the Asphenni.
A few thousand Volm left two hundred years ago, while a few billion stayed behind because they "got over it" and moved on with rebuilding their planet.
The entire population mobilized, and left Volmworld empty.
There might be a reason for that, if the ecological destruction of Volmworld was so excessive, that even if there can volmaform their planet back into a fit and liveable state, it still might take a few hundred years till it's in a fit and pristine condition.
(No I haven't seen Wall-E too many times.)
Ergo, the Volm are killing time till their world is ready.
This is all conditional to the scope of Cochise's truth.
If Ben Hathaway surrendered, which would have been the responsible thing to do, to surrender as often as loudly as possibly so that terms for the continued survival of the human race could be argued for... Is he really the President any more?