Because we’re both Light incarnate and we’re dead.
Allow me to explain. Ghost resurrected us back to life, and we have no memory of who we were before our death, yet some “spark” of us was reclaimed by Ghost and put back in the body. We’ve been blessed with the Traveler’s Light, and while it can be corrupted, the Light that the Hive eats is more or less “passive” Light. They can’t eat the “active” forms of Light, like Solar, Void, or Arc – we Guardians channel that energy. We’re not passive receptors but active wielders. Also, because we have the power of Light, that puts us on a more or less equal footing with the Darkness’s minions, including Oryx. Our precausality is what helps.
Now, if another hypothesis is true, and the “we” parts of us are actually inside our Ghosts, then we’re effectively a machine. Machines cannot be Taken, the same reason why Oryx can’t Take a dead person. The Vex had some biological matter to be Taken, but Shanks and Servitors don’t. Harpies are the only Vex that are entirely machine. You have to physically destroy the Ghost or drain its “passive” Light from it to kill it.
If Oryx could Take dead people, then why not just scan the battlefields of Mars or Earth and Take the dead bodies instead of launching an invasion? Ghost says (actually thinks, but we see otherwise) that Oryx’s power requires a living host to corrupt. See here: Ghost Scan: requires a living host.
Quria and the Tai Emperor Raven are not paracausal.
Well, I know for certain Quria isn’t, because if “she” were (using the Aurash simulation as a basis) then she could counter Oryx’s magic. As it happens, Quria discovered the paracausal nature of the Ascendant Realm and that worship was the best way to gain power from studying the worms. But no, Oryx destroyed all of her simulations and “toy worlds”, even says so himself. The Vex at this point in time are not paracausal; and still aren’t. The only places where they are is the Vault of Glass and whatever the Black Garden is. There they control reality and force us to obey those rules until we break them. Outside the Vault they are limited by what they can do. That’s why they want to rewrite reality; they want to remake the universe in their own image, either as a result of fighting Oryx or tangling with similar Hive forces, possibly Savathun’s.
The Tai Emperor Raven is just a powerful sun raven, probably with a power of her own like the pre-Darkness Vex, but I think her ability to “tickle” the gods is just metaphor. Even if she were paracausal, she was defeated by Oryx under the logic of the sword. But she did not have the Traveler’s active Light empowering her.
We do. We challenged Oryx under his own laws, defeated him on those terms, and then broke it by not consummating it.