Learn the lore. The Cosmodrome literally destroyed during the Collapse. All those cars, planes, and the remains of those that were on them were people fleeing the Fallen or whatever was attacking them. The Colony ship above Sepiks Prime was the last finished ship and was about to leave until an explosion in the fuel line, caused by rioters that couldn't get on, stranded everyone there. Thus the mass graves before Sepiks Prime. More over, not every living Human, Awoken, and Exo is within the City, also in the lore. It isn't hard to picture that your character was one of the survivors that came to the Cosmodrome to scavenge and was killed for it. And none of the guardians remember their previous lives.