I think this would be cool, but I think the narrative reason why this doesn't happen (which covers the game design reality that it would be confusing since its a shared world) is because there arn't very many guardians compared to the hordes of enemies and if we stay still they wound send bigger forces/weapons to take out our outposts. Guardians strength comes from the fact that we come out of nowhere, hit hard, weaken our enemies and get what we came for, and get out. We just don't have the military industrial complex necessary to sustain an all out war with our enemies.