All of that would make sense if the DCU operated the same way our world does.
Yeah, they saw his corpse... right after an epic war fought for the whole of existence involving gods, multiple universes, time travel, multiple versions of heroes, villains, and all sorts of craziness.
Yet it doesn't dawn upon anyone that during that whole mess the corpse they buried might have belonged to a Batman from another universe, another time, or whatever.
Applying normal laws of life and death in this case is insane.