Aliens stole their bodies
I asked myself the same question!
What Happened to Dead Bodies After Big Battles Throughout History?
Humans are bio-degradable
#6 are you sure? The bones should be there for tens of thousands of years and their artifacts as well!
#7 But the bodies are flesh - they just rot over time, eventually becoming one with the ground
#8 not really. Their bones don't rot XD
:P Yeh the bones and what they're wearing (depending on the material) don't rot, but the flesh does
#10 you are right, but these historians can't find mass graves of artifacts from great battles. They have found only small scale battlefields, but not the great ones.
#12 ahhhh okay sorry
#12 the mystery OP is asking: where are the artifacts.