I think it's because bungie intended for us to insert your own background for your character.
We can infer that we were refugees fleeing from the collapse since our body was found just outside the exodus ship launch area, and we were surrounded by cars and skeletal remains.
Although this does get more confusing if your playing an Awoken since they weren't made till after the collapse and didn't return back to earth till sometime after the 1st guardians were made.
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Explaining the Awoken bit is a little easier than you’d imagine. We were clearly one of those that chose to split from the Reef and try to assist humanity, only to be killed in Old Russia, whether by Warlords, Raiders or Fallen. Fits in nicely with the Guardian Tenants.