That's because we've all played Destiny. It is very heavily implied that our characters weren't around to know about what happening during or after the Collapse because it's all continually explained to us as though we didn't already know. We didn't know what Ghosts were. We didn't know about the Speaker. We didn't know about the Fallen. .. or the Hive. .. or the Vex. .. or the Cabal. We didn't know about the light, or even the Traveler. We didn't know about the Darkness either. How can someone who was alive during/after the Collapse not know ANY of those things?