Why we want their bodies back
As humans have evolved, they've learned there are good reasons not to bury an empty coffin. The desire for tangible proof of the death of someone we know or love is a natural human impulse. But often that desire extends well beyond a purely rational need for certainty.
In circumstances where there is not the remotest chance that someone is still alive, we still expend great energy and often put other lives on the line in order to retrieve the dead.