1. When you die in a darkness zone, you actually die, no revive, that is why you go back to the checkpoint. We are also grossly outnumbered by the other species. There are several grimoire fragments talking about Guardians dying. We are also not sure if new Ghosts are being made by the Traveler and think about how many dead Ghosts are just lying all over the place. Less Ghosts means less Guardians. 2. While each of the species fighting against our kind may be related to the Darkness, they all have different motivations. The Cabal were sent here on conquest orders by their emperor, that is why they don't simply blow up our planet, they want it for themselves, they just need to take care of the pesky species that is in their way, us. There are also records that indicate that they are afraid of something behind them. The Hive are here for the sake of their religion. They worship the god-king Oryx, who may or may not be the Darkness, and are here on his orders. I think they see the Traveler as "evil" though I think in their society our evil is good and our good is evil. So basically just think of them as Satanists. Vex also worship something Dark, in their case the Heart. The Fallen are mysterious and may have different motivations depending on what house you're talking about. So even though they are all related through the darkness, they may see the others as getting in the way of their goals. 3. The vex have not conquered everything because: they were inactive up until the collapse; They do not have a nexus on Earth yet to teleport to; Ikora says we should think of each Vex collective as a formula or an algorithm. She uses this to explain that the reason the Vex are doing so badly against the Cabal on Mars; saying that maybe that they are simply the wrong algorithm for the problem that is the Cabal. Perhaps the Vex have not found the right formula to kill us all.