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.

Back in D1 there was a setup to dive into our background. In Taken King there was some dialogue from Ghost about how we were abnormal, he couldn’t read our mind. Just like how were we abnormally strong and could beat enemies that powerful Hero guardians couldn’t. For a long time everything was canonically attributed to one Guardian then with New Light it became different Guardians and their strength was explained by the Traveler waking up and empowering Light. That was also when they started setting up The Darkness, which they admitted when base D2 released they didn’t know what it was still. So I find the timing to be the answer, my speculation is: As Darkness was originally corrupted Light, our Guardian was likely a champion of Darkness that died during the Collapse. Guardians got memory wiped so we wouldn’t know and would fight for the Traveler. Since we were strong already, we had a leg up in terms of power.