Causality. The Exo makes it incredibly clear in her journal how imprecise the jump process is during her observations of the protagonist. She'd have had to jump to get the eyes long before her encounter with us on the Moon, and at that point, it was shakey at best. You can circle around that journal, but it'll get messy and I can't see Bungie wanting to complicate matters that much, for a fanbase that struggles with the idea that two incidental characters were gay, let alone asking them to follow theoretical physics and time-travel fanwank :p