Vex locations in Destiny

While looking through lore and seeing some different things I’ve become curious about why the vex are on some planets and not others… currently they’re on Mercury, Venus, Mars, Io, Nessus, Europa, and a little bit on the moon.
I can’t help but wonder why they aren’t occupying more important planets such as Earth or the reef( though I know there are vex there they aren’t occupying it like io or mercury) or even Titan? The vex are powerful, being able to go toe to toe with the hive. And it would be extremely difficult to counter a vex invasion of earth. But they aren’t invading the very planet the traveler sit on and I’m really curious as to why? And through the books of sorrow you know that the vex like invading ascendant planes and pocket dimensions which the reef has both so why is there no pyramidion in the dreaming city or on an asteroid in the reef?

Maybe they fear Earth? It is said multiple times that we are the only thing that they can’t simulate, and maybe that makes them fear us? That could make them not want to go to the very source of that power.